Cheapest way to complete CS prerequisites (RA Credits)?

[Tomas]

I doubt you could get LOR from Metropolia professors, though of course one can always try. It is completely contact-less form of study. Also, I believe there is no grading, though someone should confirm that, just fail or pass. So that might be disadvantage in case admission committees somewhere are looking at grades of the courses passed.
Another DS and Algorithms course I think was from University of Helsinki. Might be more rigorous.
(Regarding LORs, I found it interesting that one professor teaching Gatech courses on EDX apparently gives out LORs even to EDX students, though from what he wrote, it sounded to me like "confirmation that you have absolved a course matching Gatech curriculum" or similar, so pretty uselless IMHO but still one out of three crossed with that one.)

Re Metropolia, if you did not pay instantly, they will send an invoice after some time. You can just complete Viope courses in the meantime (except those that are not on Viope, you probably will receive a list with access credentials - I am still waiting for mine and I have paid about a week ago) and send to their email for evaluation.