A ton of refined hydrogen fuel costs 500 Cr. most places. there is a lot of screaming about this cost inflation of the most common element in the damned Universe by merchant captains (military officers are used to paying ridiculous amounts for everything and it isn't their money.)
Just pay the fee.
A frontier trader (per Cepheus Engine) takes 45 tons of fuel and can travel in system at 2 gees.
45 tons of fuel costs 22,500 Cr. most places.
Refueling at a gas giant cuts that down to nothing so captains all buy fuel coops and fuel purifiers and get fuel for free.
A free trader costs @ 80 million credits (any version of CTyou play).
The mortgage amounts to 341,000 Cr a month. Life support and salaries adds another 50,000 Cr say for @ 400,000 Cr. a month.
Traveling to a typical gas giant at a distance of 5 A.U.s is going to take you 4.2 days. You ship is not earning revenue or on its way to earning revenue. 400,000 Cr. a month divided by 30 * 4.2 gives us @60,000 credits that trip just cost you. Doing it twice a month will cost you 120,000 Cr. in effect. To save 45,000 Cr. worth of fuel.
You are in effect wasting 8.4 days a month schlepping to a gas giant. Not to mention all the fun stuff that may happen to you en route or while refueling. That isn't counting the added value to the ship and the increased mortgage and reduced cargo space the refinery takes up to let you refine that stuff and be on your way.
A ship typically spends one week in transit and one week in port looking for cargo or trade. To refuel you've cut that down to 2.8 days already and in a few ports you might stuck on the end of a holding pattern. They will let the ships that buy their fuel in port land first. There's more money to be made on fuel and berthing fees. Oh, berthing fees would probably sell by the week. So spending only two days in port will not save you anything (parking garages in New York City have a similar pricing structure.)
Also, your crew expects a good chunk of leave time on the ground and you just blew it making an in system run. Expect to pay bonuses or lose crew and reputation.
Now you could leave your crew onboard to do the refueling and take a fast shuttle with your cargo to the mainwoorld. But Even if you had a vehicle that could make 6 gees and haul your cargo it's take you about 2.5 days each way. That'd leave you 2 days out of the week for brokering and hunting down freight. Plus insisted runs cost 100 RCr. per ton to haul your cargo, both ways and a few hundred for yourself.
Where this falls apart is when the main world is a moon of a gas giant or ice giant. Then a starwort might not even bother to sell refined fuel. But in that case the berthing fees would be higher to make money off the clowns skimming the giant and selling fuel cheaply. They need to berth their ships to make repairs and keep their crews sane and take on supplies. The fees would in fact almost be prohibitively high. the only people getting rich there would be the skimmers and the port owners.
So shut up and pay the fee.
Or ... take a ship in the ocean.