I will prefix this with ... "anything is possible, its code".
Now, how is your coupon configured? Do you have a rule saying customers get 50% off entire order?
OR do you have a coupon that says apple and banana are discounted at 50%?
Its important because depending on your use case, you may see different order totals if you add tax and/or shipping fees unto the mix.
Depending on your use case - implementation may differ and a well written solution should cover all use cases. It sounds like a provider you selected for the integration does not support all use cases or you are asking for an implementation unique to you only.
In either case, they should explain why they are saying "no" to your request.
P.S. If you run into any issues, reach out to me, I am happy to discuss the problem you are having in more detail and see if we can help (we have many SF integrations behind our belt)