Nope. You pay tax per item value. Each item value has to be declared. If you get credit or a voucher off the total order then it doesn't matter in the customs declaration.
Plenty of examples of people complaining on social media when they think they get free items but have to pay huge customs bills off those free items.
Unfortunately we all have tried to get Buffalo to give us a breakdown of the customs calculation for our orders but they refuse to.
Your invoice should include the actual price paid per item. You pay tax on what you paid.
When customs does not believe the invoice they ask for a proof of payment. They work from that. Proof of payment would then be a credit card statement.
Someone is doing customs declarations wrong there.
You don't pay tax on some arbitrary value but on actual cost of goods. Otherwise you should pay duty and VAT on the manufacturer's recommended price. So Temu discounts which they state for each item should not be valid and you should pay duty / vat on full price.
The discount should be spread out across all items in the invoice. They're just declaring the items wrong.