Of course the simplest thing around the world would be for crews to just whip out their Amex, Visa or Mastercard and wack every fuel uplift onto those. Unfortunately most crews are not given such credit cards (normal business account credit cards) or the ones they have don't have anywhere near enough credit on them and nor do the operators have credit accounts at each and every airport or FBO or with every fuel reseller they go to around the planet. As such, with no 'credit' available, they have to work with flight planning outfits or other specialist companies, fuel card/credit/carnet providers with whom they do have credit accounts, to pay for the fuel uplifted on their behalf, often by pre-arrangement and each person in the chain takes their percentage. Certain crews, for let's say a number of subcontinental operators, carry around huge amounts of US dollars to literally pay cash, because nobody will extend a line of credit to that particular outfit.
The whole system is extraordinarily messy, massively competitive and open to all sorts of exchange rate conversion fiddles where all fuel ultimately originates in US dollars per US gallon.