We have two common issues:

Subscribers who sign up, then want to make a change. We can up-charge them, but not update the invoice to reflect their new product.

Customers taking offers they are not eligible for. The ability for better control of coupons has been highlighted by another user. Because of these drawbacks to the coupon system, some subscribers find ways to try and exploit them. For us - this looks like cancelling every month to use a new code every month. We use terms and conditions to mitigate this, but the coupon still shows on the invoice and we have no way to edit this before dispatch.