as the CFO of openHAB Foundation, please let me answer your questions.
openHAB Foundations is run by volunteers, so we have to keep the effort as low as possible.
To follow this, we have chosen just three payment methods:
SEPA direct debit [preferred way]
PayPal [if you are not in a country of the SEPA region, e.g. USA]
SEPA payment for organisations.
Having more payment options, we (in fact just me) could not handle the accounting job besides day work.