That sucks. This is the problem with tracking, I think. If someone asks for tracking I'll add it, though it costs about £10 instead of £3.50 for tracked shipping to the US. But the problem is that it allows people to keep watching the progress of the parcel, and they panic if it stays in one place for too long, even if it's totally normal.
Are you sure customs are responsible? If customs inspects the package then the recipient will receive a fee, and if they're blaming you for how long it takes I'm sure they'd blame you for that, too.
In my experience, it takes 2-3 weeks for packages to reach the UK from the US, and I would never buy anything unless I knew I had a month or so before it was needed if it was a gift. If it's just for me then it can take all the time it needs.
It's a bit ridiculous to blame you, though. You should firmly state in your policies, and the messages, that you don't control the courier, but that things will move as fast as they can. That it's been delayed sucks, especially if customs are responsible, but there's nothing anyone can do.
If it IS customs all the time, then you might want to reconsider your customs forms and packaging. Do you state that it's vintage clothing? They might think it's a fake designer piece or something that you're not supposed to be selling. I'll be honest, I only bought one handmade garment from the US and that was the only thing that's ever been opened by customs (so far) and I buy from overseas a lot. I think they're just making sure it's not a fake designer piece or something, but clothing seems to be targeted a lot. Perhaps that fact should be mentioned in your policies. If people don't read it then that's their problem but as long as the info is accessible then it's no problem really.
Sorry this keeps happening. If you make more than enough domestic sales then stopping international isn't so bad, especially if so many of your customers behave in such a way. You're in the US, so you have an enormous audience even if you limit it to US only. Though, as an overseas buyer myself, I hate it when shops don't offer overseas shipping. It's understandable in the US, though. But it baffles me completely when I see UK shops selling small things but only shipping within the UK.