What is the best and most economical way to post a completed US tax return to America? Bearing in mind that it prob needs to have proof of postage at least.

Appreciate any advice.

A class Einschreiben. will have proof of delivery. You can also submit online.

We send ours by priority international mail, registered w/acknowledgement, and have not had any problems so far.

If the return in under 1000g, it's around CHF 25.00, under 2kg is ca CHF 50... if the return is heftier than that, it starts to get expensive.

In most cases, you can submit online. If you own rental property, it is not allowed.( for
whatever reason )

correct. That's why my CPA submits it. Postage included in the invoice he emails me

Step 1: Copy verbatim the address to which your return goes on to evelope.

Step 2: Buy a first class via airmail stamp.

Step 3: Mail.

I did this back in March. Less than two weeks later, I had a nice withdrawal from the IRS and New York State from my bank accounts, complete with scanned copies of the checks I wrote.

I didn't bother with confirmation receipt; the post office knows how to get your hard earned money to the IRS. You bet your US dollar on that!

I also mail mine by the usual Swiss A post. I never owe any taxes, since I don't make enough money, but I have called in the past to make sure they got the return-it's never been a problem (and, surprisingly, they've been really nice!)

I second this. The first couple of years I sent it A post with delivery confirmation, tracking, etc. I wasn't so zealous this year, especially as I didn't owe anything. I went with standard A post.

If Uncle Sam owes you money though, I'd definitely get all the bells and whistles.