Some considerations:
Its important to understand the US inter-state taxation: Sellers have to pay sales taxes applicable to the state of the buyer when they have a presence in that state. Which is why reshippers like viabox have addresses in states with 0% sales tax. So the seller can be anywhere. There is no way to get sales tax back once paid (unlike shopping in europe).
Wrt: mail-in rebates: You first have to send in the mail-in rebate form. Hopefully nothing happens when the processor sees that it was shipped from overseas.
Then they will nowadays most likely send you back some prepaid debit card, typically from amex (i also had visa/mastercard). Checks was in the past. Maybe they still exist, i have not seen any in the lat three years. That letter with the debit card would be another letter that your forwarder has to pass on to you (cost).
Then you have a prepaid US dollar debit card. No idea how easy it is to spend that in Europe. They may not work overseas at all. I can try to take that $250 Amex i get for my Pixma Pro 100 to Europe and try, but that will take a few months to tell. And they typically expire after a year, so if you intend to spend on vacation in the US, make sure you remember that.
The discounts of frequent shippers are certainly worth considering. Even in the reverse direction: In 2012 i bought a 15 Kg shipping weight projector from a small home theater store i Germany, and he shipped it to California for about 1/3 the price i was quoted by UPS ($100 vs close to $300 if i remember correctly). And of course i didn't have to pay VAT in Germany because the store & UPS marked it for export, and USA customs is always very low.
Which basically brings me to the last part of international shipping: I really do not want to have anything shipped to Germany anymore. German customs is just a fre#&%*&#%^$ shi$% he#$ of as#$(&^$*% fu$%*&^ CENSORED! I'll stop here or i get kicked off the forum
