Hello, the same thing happened to me. Also the price showed was 1890 coups, the pop up confirmation also showed 1890, but i was charged 2700 coups. I sent a report for this visual bug from the mini client as well with some pictures.

S142:Utakata
UID：200000089708178
i can atempt to recreate the bug, heres a pic, where you can see that the prices are discounted, but the voucher isnt checked. I realise it's a mistake on our part (players) for not paying attention to the vouchers, but it is also a visual bug where even the double confirmation shows the wrong price :(.
Also had plans and had been saving the coups so losing 810 coups on a visual bug hurts :D