Dynamic Member Tags on Checkout

Quick question,

On my site we are offering separate courses for users at various prices. I was going to put the content of these courses behind a paywall that is only accessible via a members tag.

I realise I need to add the perch_member_add_tag to the result master page

something like

if (perch_shop_order_successful()) {

// Yes! Show a success message
perch_content('Order successful');

// Redisplay the details of the order
perch_shop_order_items(
    perch_shop_successful_order_id()
);

perch_member_add_tag('TAG HERE');

perch_shop_empty_cart();

}

This sounds straight forward, what I cant figure out is how to add a member tag based on the course they buy, i.e Dynamic Tags.

Any ideas how I would go about this?