How to add coupon to Appointment+ without using Marketpress cart ?

Hello,

I would like to use coupons in my Appointment+ installation.

I also installed MarketPress, but for many problems (actually incompatibility with WPML and other usability features) I would not like to use the marketpress cart.

What I need is indeed very simple: add a coupon field directly in the appointment form, so that the user book the appointment, enter the coupon and go directly to the paypal payment form.

Is this possile ? Does it require custom coding ? If any ... can you direct me in the right direction ?

Thanks for your help

Antonio