I had a package shipped to me from Glasgow about a month ago. It wasn't a kilt, but had kilt hose, sporran, etc.

In total, it took about 10 days for the package to get to me from the postal stamp on the envelope, sent through the UK's Royal Mail, and then through USPS here in the States. There was a sticker on the package where the company had marked that it was a "gift" being sent, so I think that saved time/money for both of us.

I wasn't notified by customs at all. There was just a sticker saying "Approved by US Customs Dept." on the package.

I don't know if kilts will be another story. That's what I learned from my experience.