Changing Invoices In Skrill Gateway

The bunny deep inside me is not well-chuffed. Several times recently I have bought stamps from dealers who add an aditional payment in expectation of settlement by Paypal but offer a reduction if you pay by Skrill.So I chased the spiders away from my Skrill account and prepared to pay. I then found that the Skrill Gateway will not let me adjust the invoice (unlike ye olde times.)

So where the sum involved was considerable I had to email the dealers and ask for a revised invoice, thereby wasting my time and theirs; where it wasn't a lot I just gritted my teeth and paid by Paypal instead. Which is presumably not what Skrill have high up on their Mission Statement.

So am I very stupid and missed an easy way of editing? Or are the dealers failing to press a simple button that makes the invoice editable? Or is it just Skrill again improving things until they are much worse?