IMTU, Ship's payments are documented on "paper", the ship's transponder, and notice forwarded via X-mail; most A and B ports will have enough to justify a daily payments X-mail (authenticated and one time pad encrypted). The transponder has an encryption-cypher taken from a standard book for the company issuing the loan; twin key cyphering, and if not reset within 35 days, starts "squaking" "Skip" rather than the normal return on transponders.
Of course, such transponders are often tampered with... but f one gets CAUGHT, it's a high crime. (IMTU, Barratry, punishable by DEATH.)
I assume that most often, if one want's a credit account good on multiple worlds, one gets letters of deposit through the banks or scout services; these letters are, semi-negotiable instruments. They are not transferable, being issuee only, and are a guarantee of X amount of reserved capital; only a bank may negotiate it, typically. If they send a notice of funds expended, and recieve back the right funding(taken from excess in the account above LOD face value), they'll return it to the issuee after all is handled. If there are not sufficiennt funds, they send the instrument via courier or XMail, to get the FULL letter face value. IMTU, most merchants try to keep a LOD for KCr100, just so that they have a full run, just in case.
Basically, it's a means of getting a loan against a non-accesable deposit in a different system... I usually charge characters 0.01%, minimmum Cr100 for LoD's of KCr1 or more.
Under 1KCr, traveller's transit cards are available from the IISS. It's a bearer card, with an imprinted value, and a holo of the bearer. Cost is Cr10+ face. most local banks treat these same way, charging Cr5 for a local debit card, with same value, as a prepay. They cash them with the IISS.