I've been wrong before and I'll be wrong again, but I think I might have a bug.
If trade products are set exclusive of VAT, then I think tax is added to the shipping amount when trade is enabled even though the amount you specify should include VAT according to the UI. I odn't think there's a simple way around this if you want to share one shipping for trade and non-trade.
Am I doing something wrong?
Perch Runway: 3.1.4, PHP: 7.1.23, MySQL: mysqlnd 5.0.12-dev - 20150407 Installed apps: content (3.1.4), assets (3.1.4), categories (3.1.4), perch_blog (5.6.1), perch_forms (1.12), perch_shop_orders (1.2.6), perch_shop_products (1.2.6), perch_shop (1.2.6), perch_members (1.6.4), perch_mailchimp (3.1)