I went to Office Depot and bought a bunch of multicolored mailing envelopes with the bubble wrap in them (less than $5 for 6) and use them for so many different things (you can send them priority). They are really nice to have hanging around the house when you are getting ready to ship things out.
I use flat rate boxes when I know everything won't fit in the envelope. I can really pack a box when I have to.
I've found shipping from the US to somewhere else is not the ordinary day. UPS tracks ground shipments but charges an arm and a leg, you can get tracking at the US Post Office and it will cost an arm and a leg but you have the option of just mailing it and it cost about 1/5 of what you pay for UPS. I didn't check into FedEX this year. Maybe another day.