I run my stuff across the border.
You're going to need a form 7523 from US Customs. It's pretty simple to fill out and it allows you to take up to $2,000.00 across the border every day as long as there's no duty payable on it.
If you have dutiable items, then I would strongly suggest that you use a customs broker - Fedex Trade Networks seems to have their act together and I'd suggest you start there.
What I did was I did a dry run through the border - I went to my border point, went into the Customs office and told them what I wanted to do and asked what they recommend. They were very helpful.
You combine all of the invoices into one clearance form - you are the importer, there needs to be no receiver.
One thing they STRONGLY suggest is that you copy each order coming across and attach it on the back of the customs form. I have done so ever since ( ebay has a bulk print option for this) and I've had no problems.
It can cost $10.75US to do the informal clearance, but they don't apply it every time. Your car also gets X-rayed fairly frequently, and occasionally they do an inspection on you.
I've found that you have to set up a US Post office box (minimum size) to use USPS online. It's worth the $60 per year to do it, you'll save that almost immediately.
I would also suggest that you look at endicia dot com - this is a popular service that allows you to buy stealth postage at about an 8% discount from regular prices and gives you delivery confirmation either free or very cheaply. It's very easy to use. You'll need a US-based bank account and a US based credit/debit card for that (in the US, bank debit cards are also credit cards)
Be sure and check out the Priority flat rate boxes - they're an exceptionally good deal for long-distance shipping, although it can be cheaper to send by regular Priority Post for light stuff and close stuff.
Also check out First Class Mail - it's a Godsend for light pieces. There are different restrictions and weights for domestic and international - there's a learning curve, but it's worth the effort.
Did I mention that USPS gives you FREE BOXES for Priority post?
You do NOT need to set up a company in the US to use USPS - a post office box will do. Small business in the US has special problems - the States have onerous reporting requirements ( you need an accountant to fill out their reports - takes about a week every time) and many states are on the verge of massive tax hikes because they're all in trouble down there.
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"To be nobody-but-yourself -- in a world which is doing its best, night and day, to make you everybody else -- means to fight the hardest battle which any human being can ever fight; and never stop fighting." -- e.e. cummings
Imagination will often carry us to worlds that never were. But without it we go nowhere. Carl Sagan