This is related to a question on the Interpersonal Skills site about refusing free goods from shopkeepers, etc., and paying for them instead. (Note: I'm not the OP of that question, I'm not blind or in Germany.) The most popular answer now suggests a polite but firm "I insist!", and that is what I'd probably use if the conversation was in English. But in Japanese, I'm not so sure how to say this. Would something like「失礼{しつれい}します、お支払{しはら}いたいです」be fine?
It's not uncommon to get something for free as サービス in Japanese stores, especially if the shopkeeper made a mistake in something. In such instances, I'd still like to pay for whatever it is they are offering me, especially so if that's what I wanted in the first place.