Weird that they'd be that different than PayPal. My experience with PayPal was that my credit union savings account was valid for credits/debits ("transactional" per
https://squareup.com/help/us/en/article/3896-link-and-edit-your-bank-account).
Have you asked your credit union for savings account-specific routing/account numbers to give to Square, or if only certain credit unions have a workaround for savings accounts?
In the end I closed my account because of
Reg D. I wonder if the reg changed in the last few years to make all savings accounts, or credit union savings accounts, non-transactional.