I am trying because Click-N-Ship does not allow First Class Mail but do not have credit card with US address, which Endicia requires. Even my M&T account has Canadian address.
Although I did use my M+T card initially with Endicia (and had zero problem doing so) they made me switch to direct bank debit (ACH) because of the high ratio of First Class (Domestic & International) that I was using. Endicia makes no money (actually lose money) when customers use a credit card to buy postage for First Class (Endicia gets no extra discount from USPS for FC and USPS don't pick up any of the credit card fees).
Just switch your payment method to direct debit! Regardless, Endicia don't give a hoot what your billing address is but they do need a US address to establish your sending zip code (also will be your default return address on labels).
Even the default return address that shows on the labels can be overridden with a little ingenuity. The important part is that you mail from within the same zip code which is part of the Indicia.
If my memory is correct if you are in the US for more than 180 days the IRS considers you a resident
That would be 180 complete 24 hour days, not 180 visits of less than a full day.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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