Yep, along with SteveM example, if I process sales this week of $11,000 and get charged 3% I am debited $330.

If $1,000 of that $11,000 comes back next week for credit and return, I process $11,000 of my new sales and a credit $1,000, so bank charges me 3% of only $10,000 or $300.

So yes I did pay $30 in Scene #1 but have it offset against banking in Scene #2. Zero sum game in the wash-up.

They cannot charge you 3% on $12,000 in Scene #2 .... double jeopardy for merchants then!