A Marketplace Store is the only University-approved method for accepting payments online. The great news is that a Marketplace Store is free*, easy to setup/modify, and allows 24x7 access to view incoming payments. The site is robust enough to handle conference registration for events such as NCUR and UBUG, but easy enough to use for a simple site to collect membership dues or key deposits.
*As with all credit card transactions on campus, the department will still be responsible for the 3% merchant fee for processing a credit card. The University does not make a profit on this 3% fee.