If you first entered a Canadian address, checked the shipping options, then "back-paged" and entered a US address, the automation likely was messed up somehow.
We have made a "$0 shipping" option for those needing to quickly add to an order (where they've already paid for shipping)...and that option should always be available...but it should never be the ONLY option.
Generally there's either one option given (with a drop-down arrow beside it, to see/choose) the other options.
If you'd like, just complete your order (putting in the address you want the parcel shipped to) - then follow through the checkout process until your order is complete (and you've received an email saying so)...and then I will go in and adjust the invoice to be sure you get the lowest possible shipping. :::sigh::: there's no way for the automation to determine exactly how much will fit into the various flat-rate boxes (which are weight-based, but VERY limited as to the size of the box).
I should be here for the next 4-5 hours to do that - but I'll be taking a couple 30 minute breaks away from the computer to help Rocky with inventory.