Use Customer's USPS Express Mail Account

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Some of my customers have their own USPS Express Mail account for shipping option. I modified the In Store Pickup shipping module to read "Use My USPS Express Mail Account", price is 0.00, of course, as they pay later, but they have to enter their Express Mail account number somewhere. I have some text that says "Please enter Account Number in Comments Box", but was wondering if there is a way to have a text input box in a shipping module for the account number. Something like the Purchase Order contrib has, if you have seen that. Also, it may be helpful to have an entry in their My Account to store this information and then auto-fill the text box for them. Anybody? TIA - Joe