This is one of the things where mathematics gives way to linguistic manipulations. If you add a "late fee", it's a no go. But if you get an "early payment cred

This is one of the things where mathematics gives way to linguistic manipulations. If you add a "late fee", it's a no go. But if you get an "early payment credit that will appear on your next invoice", it is ok. Don't know if this is still the case, but for a long time, merchants couldn't add a "credit card fee" (even though they pay it at 2-3%). However, they could give a "cash discount". Mathematically (and financially) equivalent. Yet somehow different.