Thank you for your question. I love this concept and it is a huge help for businesses where they are able to make more profit rather than having to pay more to credit card companies.
It would be great to get some more background regarding the clients you currently/whether you have a specific target market. This way it would give me an idea as to how to appropriately market your business.
With a lot of businesses online nowadays, Facebook/Instagram advertising could be one way to get your business out there and have it more well known. This is more of a generic way to gain traffic to a website or to an email subscriber list.
Again, getting some more background of a target market would be beneficial as then I would be able to help market. Another way could potentially be using Google Advertising and keywords so that your business information comes up when keywords are entered.
I would be extremely interested in helping you with this. I run eCommerce stores myself and are very aware of the fees that credit card companies charge. I also have a client base which with the help of word of mouth, I could promote your business as well.
Please feel free to get in touch with me firstname.lastname@example.org otherwise hopefully some of these ideas help