Round turn commission in addition to spreads can be offered in the cash market, but not with many.
Generally its just a spread and the "round turn commission thinking", needs to be forgotten, as most Retail Spot Forex dealers/marketmaker do not charge a fixed round turn commission.
Thus you get all of the misleading advertising...like "free commissions"..."free trading" and the like in the Wild West of Retail Spot Forex.
A fixed amount verses a PIP spread, that essentially is a percentage, is better for larger size obviously.
Michael B.