How often does this happen? What is it costing your friends? Are they sneaking it to you? Are they the purveyor of the establishment?

In any case, if you were only going to spend $5, you don’t have to accept the full $20 worth of food/drink. You could say, “oh I was only planning on spending around $5 tonight, this is too much, but thanks.” Or if the somewhat direct approach feels awkward, max out your tip at what you were originally going to spend. If it’s not enough, they’ll stop giving you so much.

Well it finally changed the 8th time I pressed it, so checkmate.