The person not including it in their cost can sell for cheaper. That's a competitive advantage. Or if they don't sell for cheaper that's 3-5% higher margins.
Discussion
Just mark up by desired margin + fee…
Then it's more expensive than the person who doesn't have to markup.
Compete on quality and branding? Which industry is so competitive where a 3% difference will put you out of business?
Its like you're promoting running a race with a bullet in the foot. Why would you want to do that?
There are a lot of industries where this is the case. Most physical retail stores for example.
A 3% charge from the credit card company is just as bad as 3% inflation. If the charge can be avoided it would be in the interest of the merchant and the customer to do so.
Also it might make sense to you because you are thinking of freelancers, but think of the exact same product, same label could be sold to you at 3-5% less.
It kind of balances out anyway. You charge me more, I charge them more 🤷♂️