3% is probably inaccurate, it may be 2% with a cap of 21 cents or something like it, I don't know the average percentage. I do know it is upsetting enough to Walmart to sue for 5.9 billion in 2014[1] and join a class action lawsuit for 3 billion in 2003[2], to go in on a electronic mobile payments processor with rivals target and stores rite aid and cvs[3]. I got all this perspective following the news that Walmart would not back Apple Pay because it was holding out for it's own system [4] with lower fees called "CurrentC" (that I think died? I haven't heard about it since).

That last article quotes it at up to 3% but most say 2%, so I am assuming there is a percentage and a fixed cost, and possibly a cap. In any case Walmart had a long history of being very unhappy with the relationship.

[1] http://mobile.reuters.com/article/idUSBREA2Q2BJ20140327 [2] https://mobile.nytimes.com/2003/06/06/business/merchants-may... [3] http://www.computerworld.com/article/2839144/heres-why-rite-... [4] http://time.com/money/3541247/apple-pay-walmart-current-swip...