We sell almost entirely to businesses and they want to use a PO and often their own UPS or Fedex account # for the shipping, any suggestions on the best way to do this? I was able to create a custom payment method under manual payments fairly easily and call it "Purchase Order", but I couldn't figure out how to have a space for the customer to enter their PO number. There was some mention on an old topic of turning one of your address lines into a space to enter a purchase order number, not sure how to do this and I'm hoping there is a better option. We also need a line for the customer to be able to enter their own carrier account #, especially for international shipments it's nice to be able to just ship on their account.
You can capture that information on cart page and then it will all appear with other order information. It is not wise to create it as a payment method.
If you can open us a staff account for firstname.lastname@example.org we can add it for you too. It is quite easy. Please send the link of this page as a reminder hooe this helps!
If you are happy with a manual way of working with this, then you can use an app to collect the UPS, FedEx credentials and also the PO number. Once these are collected, you can put those account credentials in the Multi Carrier Shipping label app and use it for fulfilment using the customer's account. Otherwise if you are looking to automate this, like once the credentials are provided, you want to automatically enable this in the shipping app, then may have to go with a customisation of the app.