How can we accept POs for payment in an online store?

[SportsAccs]

We would like to be able to accept POs from businesses so they don't have to use a credit card and pay immediately for online orders, but we're not really big enough to upgrade to Shopify Plus.  Are there any apps available to enable PO information for payment?  When I search the app store I only find apps for generating POs for us to use with our suppliers.

Thank you in advance.