And what happens next when (if)BTC tanks? they will be paying more BTC, or they will have to re-evaluate the situation every time a major change in price happens which isn't very practical. There is really no easy "fix", you pay in fiat - you are subject to pay more BTC, you pay in BTC - you become subject to pay more fiat, which why I think most businesses will pay in the "currency" they earn in order to keep their budget in place since Chipmixer is one of the few companies on planet earth that earn and spend only in BTC it will be hard to peg the payment to fiat, on the other hand, fiat wise while BTC price is at levels is a lot, and Chipmixer can really save a lot by reducing the payments at least temporary, and as Suchmoon said, most likely nobody will complain.

With that being said, and given how successful Chipmixer is and also how long they have been in the business, I bet they are more capable than most of us in making decisions related to advertising campaign budget.