Hello when I fitted the K&K big twin I contacted them through their website and ordered special lengths I wanted to shorten one and lengthen the other in order to place them a third from top and bottom, it didn’t cost much for the special order, I can’t remember exactly how much, it was in the region 10 to 20 USD. I had no problem with customs and I live in France, I had to pay VAT to to the postman, but tax is tax and I would have to pay that wherever I bought them. They arrived pretty quickly within a week. I am still happy with the K&Ks on the Harp, they’ve been on for a couple of years now and I haven’t had to do anything, they just work.
I have used other cheaper (eBay) piezos, I build a hammered dulcimer during the lockdown and when I started playing again when health restrictions relaxed, it also was too quiet for the group , my experience with these cheaper piezos for the HD was not so good I need to use a preamp with these, so in my opinion it’s better to pay a bit extra and get good ones.