i'm in the US and ship international with some regularity.
1st class is slow, slow, slow.
priority should be a week or so...it goes by air. however, there is no insurance and no tracking/delivery conf. unless purchased as additional services, and i believe the tracking is only available to USPS employees in case a box goes missing...ie: you can't watch its progress to you online.
express int'l includes both real-time website tracking and insurance.
priority is fine for small bits, but for a frame, i think i'd want the extra reassurances provided by express.
fwiw.