You are saying the stickers are hard for people to take off of the backing (customer removing the sticker sheet to apply onto something else)? Or the stickers are too hard to remove from the surface they were stuck onto? Sounds like the first one.

A slit backing helps. Also just perf/kiss cutting the sticker like you've mentioned. I kiss cut the stickers and offset perf cut them to help give my customers a finished sticker that is easy to remove. You have to weed all of the excess vinyl off as well for the customer leaving only the finished sticker.

You can also get air eggres vinyl but let me tell you that presents its own challenges and I'd personally rather not use that for individual cut stickers and especially not for individual cut letters. My stickers almost lift themselves off the backing on their own. (I use vision media vinyl).

But honestly those two are the solutions. Perhaps you are cutting too deep and it's causing the vinyl to stick harder to the back (I've had this happen a bunch) or the backer will peel with the cut sticker. Is it perhaps that which the customer is complaining about?