Yes, it's OK to alter it and yes, it ships at the regular rates.
You don't have to be very precise with the dimensions. The box size only matters if the package volume is over 1 cubic foot (1728 cubic inches) and none of the current free Priority Mail boxes are over that threshold.
I have altered USPS boxes for years and never had an issue.
My favorite trick was making a 1095 into a square so a helmet would fit in one. The process took less than 30 seconds and voila a square box with out the need to use a larger #7 box and packing materials.
Yes, as long as you are not using the Flat Rate Priority Mail boxes. I tried that once without knowing that it was not allowed. My postal clerk explained to me that it was fine as long as it was any other Priority Mail boxes and it was being shipped via Priority Mail.
In a pinch I have modified flat rate boxes. The secret is to use priority mail stickers and cover anywhere that says Flat Rate (including the bar code on the box), then weigh and measure the box and then ship that way.
If I use that box to ship something can I resize it? Also do I simply treat it as any other box that goes Priority? (measure it and weigh it)?
Yup, I do it all the time. I've even combined two together to make an extra long box. Ironically, I got the idea by watching the clerks at my local post office helping people pack their boxes.
disneyshopper
