Using Coupons with Recurring Paypal Subscriptions

I’m using the MarketPress plugin with Paypal to sell subscriptions. To do this I’ve created several PayPal Subscription buttons and pasted the email link into the External Link field.

I need to add in the ability to accept coupons. However, MarketPress coupons only work on items in your shopping cart and external Paypal links bypass the shopping cart.

How could I add in the ability to take discounts?