Dropbox Shop is cheaper than Gumroad-- they only pass on the 2.9% + $0.30 per transaction fee from their payment provider. The way Gumroad's tiered fee schedule works, you only get those rates from them if you've sold $1 million worth of product through them.
On the other hand, Dropbox doesn't have anything like Gumroad's Discovery page, so if that's something that you think might drive sales, it might be worth paying Gumroad's higher fees.
On the other hand, Dropbox doesn't have anything like Gumroad's Discovery page, so if that's something that you think might drive sales, it might be worth paying Gumroad's higher fees.