That’s only a restriction for the built-in Coupons functionality.
Outside of that, as a developer you can add many discounts to order items, or separate items with negative totals, through the
As a non-native English speaker, coupons and vouchers sound similar, but if I’m correct, the idea of a “voucher” is that it’s more like a payment method than a discount; perhaps it’s a card people bought in a store and gifted, and its entire value should be spendable on one or multiple orders. A prepaid amount that can be redeemed as if it were cash. GiftUp looks like that as well. And a “coupon” is a discount that has not been previously paid into by the customer, but is simply a special sale that is unlocked.
From a taxation point of view, applying a “voucher” should not change the order totals or taxes (which a “coupon” would) - only the amount due. So that makes it technically a payment method (where the transaction amount is limited to what’s on the voucher and not already spent), which allows the books later to reflect the money came from some interim account that was paid into earlier.
Not in the short term.
The GiftUp API looks straightforward enough, so sure, that could be built.
Building such an integration would require quite some in-depth knowledge about Commerce, more than any other extension that is currently available.