What does "2.9% + $0.30" actually mean?

On a $100 sale, you pay $2.90 (2.9%) plus $0.30 flat, keeping $96.80. The percentage covers interchange and processor margin; the fixed $0.30 covers authorization costs. The flat fee makes this proportionally expensive for small transactions — on a $5 sale you pay roughly 9% effective rate — so it favors higher average ticket businesses.