The percentages aren't quite additive like that. Say Epic gets $1 per sale, and the consumer pays $1.3 with the 30% Apple cut. A discount of ~23% (1/1.3) gets the sale price back down to $1. Include chargebacks and such for the missing ~2%.

That 30% includes payment processing. Taking into account the probably relatively high chargeback rate (I'm thinking of kids using their parents' card), that might make up for 5% or more. So they may be taking a little off the top, but not a ludicrous amount.

I think it would be a rough percentage that covers credit card fees in most circumstances. Perhaps they are making some small percentage but it's probably closer to 5%?