UPS Stores & I believe Fedex has something similar will give you something like a post office box with a street address that you can use for a fee.
Lots of small, at-home businesses use them so that they don't have to give out their home address.

UPS Stores are small, independent shipping facilities that are located in shopping malls - the big UPS sort facilities won't accept shipments from the post office.

Our shipments go out through Royal Mail then through customs then through the Post Office.

If you want to break down the orders into smaller packages, you'll have to order separately & pay the postage with each.
We don't have anyway to do what you suggested through the system & I'm afraid it would confuse our shipping people to no end.
Plus, the mailing costs would mean that we'd lose money on your order.

I don't think you'll have any problem with 15 seeds coming through. And, most of the time, they'll replace any seeds that go missing in the mail at least once.
If seeds go missing at a high rate meaning it's a customs issue, they'll stop shipping to that region for a period of time.
So far, we've only had issues like that in Australia, New Zealand & Hong Kong.

If you look around the forum, you'll see that we've only had one problem here with someone ordering into the US and that was a training issue with our shipping department. They received their replacement without a problem.