Some schools only take money for the section of the groundschool you are about to sit or for the flying you have already flown. That keeps money secure (note I lost some when SFT went, so have learnt!). Point this out to OATS. If they do not offer you security without the cost of a card transaction (and 3% sounds excessive, I'm not sure we charge anything) then go elsewhere. They are not the only choice out there.
You should also clarify what happens if you have not finished your course. I assume from the price you are thinking of going integrated (might I ask why? No-one has ever been able to tell me an advantage to a self-sponsored student of an integrated course to offset the cost and lack of hours on graduation). One student has posted here after not being allowed to finish an integrated course, having failed a section, and was not sure of the status of his training, whether he could complete elsewhere or if he had to qualify for modular training. He never reposted any conclusion to the story, so I do not know what the answer was.