Perch Shop Checkout with PayPal Express

Hi,

I'm trying to call PayPal Express through the perch_shop_checkout function as below:

  if (perch_member_logged_in()) {

  // your 'success' return URL
  $return_url = 'https://mysite.com/payment';
  $cancel_url = 'https://mysite.com/';

  perch_shop_checkout('paypal-express', [
    'return_url' => $return_url,
    'cancel_url' => $cancel_url,
  ]);
}

We've set-up the api details under perch/config/shop.php, but not the testing ones yet.

However, when calling the checkout, nothing happens, but I've left debug mode on, and there is an error related to shipping (order shipping total cannot be null):

Debug Message
[1] SELECT * FROM perch2_members_sessions WHERE sessionID='a10e098d97fe1075c6ba97dc5795289b4f6958ba' AND sessionHttpFootprint='5e2ba474617694890918708760dff203f74e6d0d' AND sessionExpires>'2016-05-20 16:23:48' LIMIT 1
User is logged in
[1] SELECT * FROM perch2_pages WHERE pagePath='/shop/checkout.php' LIMIT 1
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
[10] SELECT regionKey, regionHTML FROM perch2_content_regions WHERE regionPage='/shop/checkout.php' OR regionPage='*' ORDER BY regionPage DESC
Checking out with paypal-express
Member ID: 1
[1] SELECT * FROM perch2_shop_customers WHERE memberID=1
[1] SELECT billingAddress, shippingAddress FROM perch2_shop_cart WHERE billingAddress IS NOT NULL AND cartID=8
[1] SELECT * FROM perch2_shop_addresses WHERE customerID='1' AND orderID IS NULL AND addressSlug='default'
[1] SELECT * FROM perch2_shop_addresses WHERE customerID='1' AND orderID IS NULL AND addressSlug='shipping'
------------------------------ Calculating cart ------------------------------
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
[1] SELECT * FROM perch2_shop_tax_locations WHERE locationID=1 AND locationDeleted IS NULL LIMIT 1
[1] SELECT locationID FROM perch2_shop_tax_locations WHERE locationIsHome=1 LIMIT 1
[1] SELECT * FROM perch2_shop_tax_locations WHERE locationID=1 AND locationDeleted IS NULL LIMIT 1
[1] SELECT * FROM perch2_shop_customers WHERE memberID=1
[1] SELECT * FROM perch2_shop_currencies WHERE currencyID=47 LIMIT 1
[1] SELECT COUNT(productID) AS product_count, SUM(itemQty) AS item_count FROM perch2_shop_cart_items WHERE cartID=8
[47] SELECT DISTINCT settingID, settingValue FROM perch2_settings WHERE userID=0
Tax mode: exc
[4] SELECT * FROM perch2_shop_cart_items WHERE cartID=8 ORDER BY itemID ASC
[1] SELECT * FROM perch2_shop_products WHERE productID=17 AND productDeleted IS NULL LIMIT 1
[1] SELECT * FROM perch2_shop_tax_groups WHERE groupID=1 AND groupDeleted IS NULL LIMIT 1
[0] SELECT r.rateValue FROM perch2_shop_tax_group_rates gr, perch2_shop_tax_rates r WHERE gr.rateID=r.rateID AND gr.groupID=1 AND gr.locationID=1
[1] SELECT * FROM perch2_shop_products WHERE productID=13 AND productDeleted IS NULL LIMIT 1
[1] SELECT * FROM perch2_shop_tax_groups WHERE groupID=1 AND groupDeleted IS NULL LIMIT 1
[0] SELECT r.rateValue FROM perch2_shop_tax_group_rates gr, perch2_shop_tax_rates r WHERE gr.rateID=r.rateID AND gr.groupID=1 AND gr.locationID=1
[1] SELECT * FROM perch2_shop_products WHERE productID=15 AND productDeleted IS NULL LIMIT 1
[1] SELECT * FROM perch2_shop_tax_groups WHERE groupID=1 AND groupDeleted IS NULL LIMIT 1
[0] SELECT r.rateValue FROM perch2_shop_tax_group_rates gr, perch2_shop_tax_rates r WHERE gr.rateID=r.rateID AND gr.groupID=1 AND gr.locationID=1
[1] SELECT * FROM perch2_shop_products WHERE productID=6 AND productDeleted IS NULL LIMIT 1
[1] SELECT * FROM perch2_shop_tax_groups WHERE groupID=1 AND groupDeleted IS NULL LIMIT 1
[0] SELECT r.rateValue FROM perch2_shop_tax_group_rates gr, perch2_shop_tax_rates r WHERE gr.rateID=r.rateID AND gr.groupID=1 AND gr.locationID=1
[1] SELECT billingAddress, shippingAddress FROM perch2_shop_cart WHERE billingAddress IS NOT NULL AND cartID=8
[1] SELECT * FROM perch2_shop_addresses WHERE customerID='1' AND orderID IS NULL AND addressSlug='shipping'
[1] SELECT z.* FROM perch2_shop_shipping_zones z LEFT JOIN perch2_shop_shipping_zone_countries c ON z.zoneID=c.zoneID WHERE z.zoneActive=1 AND z.zoneDeleted IS NULL AND (c.countryID=236 OR zoneIsDefault=1) ORDER BY zoneIsDefault ASC LIMIT 1
[1] SELECT * FROM perch2_shop_shippings WHERE shippingDeleted IS NULL
Rejecting shipping method ‘Standard Delivery’ because: no price available for chosen shipping zone
[nil] SELECT * FROM perch2_shop_promotions WHERE promoFrom<='2016-05-20 16:23:00' AND promoTo>'2016-05-20 16:23:00' AND promoActive=1 AND promoDeleted IS NULL ORDER BY promoOrder ASC
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
[1] SELECT * FROM perch2_shop_cart WHERE cartID=8
INSERT INTO perch2_shop_orders(orderStatus,orderGateway,orderTotal,currencyID,orderItemsSubtotal,orderItemsTax,orderItemsTotal,orderShippingSubtotal,orderShippingDiscounts,orderShippingTax,orderShippingTaxDiscounts,orderShippingTotal,orderDiscountsTotal,orderTaxDiscountsTotal,orderSubtotal,orderTaxTotal,orderItemsRefunded,orderTaxRefunded,orderShippingRefunded,orderTotalRefunded,orderTaxID,orderShippingWeight,orderCreated,orderPricing,orderDynamicFields,customerID,shippingID,orderShippingTaxRate,orderBillingAddress,orderShippingAddress) VALUES('created','paypal-express','20.50','47','20.50','0.00',20.5,NULL,'0.00',NULL,'0.00',NULL,'0.00','0.00',20.5,'0.00',0,0,0,0,NULL,'0.00','2016-05-20 16:23:48','standard','[]','1',NULL,NULL,'1','2')
Invalid query: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'orderShippingSubtotal' cannot be null
[1] SELECT * FROM perch2_shop_order_statuses WHERE statusKey='created' AND statusActive=1 AND statusDeleted IS NULL
[nil] SELECT * FROM perch2_shop_emails WHERE emailStatus=1 AND emailActive=1
Array
(
    [type] => 8
    [message] => Undefined index: shipping_tax_rate
    [file] => /Users/alexbennett/Desktop/Job Folder/Cooks Lane Herbs/New Shop Site/admin/addons/apps/perch_shop/lib/PerchShop_Orders.class.php
    [line] => 69
)

Should I be adding a shipping function before calling checkout to add a value here? Sorry if this is a stupid question. I've set-up two shipping methods, one standard and a free one for over a certain amount.

Thanks, Alex