To receive payment from a new customer, they want us to "participate in an electronic transaction program using either Ariba Interactive Email or the Ariba Supplier Network (“ASN”)”. Does anyone have experience with this / know what’s involved?

Ariba is an electronic payment platform in use among many large companies.  I’ve used it at a previous job.  It is sort of outdated and expensive, but there are still many, many large companies that use it.  At a high level, it just involves giving Ariba your banking info that allows you to write electronic checks and for buyers/sellers to present PO’s and invoices.  The biggest enterprises use it. Ariba is part of SAP and quite often they’re sold together. For suppliers/vendors it’s free of charge until you hit a certain limit in dollar value of POs in a year, then you will have to start paying a % of every transaction. They lure you in, but once you start transacting in it, there’s no way to get out anymore