If they are coming into the country from outside the EU you may well have to pay the customer and exercise mob import duty and VAT and the post office about 8 quid to collect the due amount. Depends on value (including the postage). It used to be about 25USD limit. Don't know what that happens to be nowadays. I recently got clobbered for an extra 120 quid for an item recently. I thought I had been overcharged by a relatively small amount, so I politely asked customs to look at it again. Got a very nice surprise when the reply eventually came back - the best part of ninety pounds! Probably the age of the item made a difference, I don't know. I was over the moon as it made the item sooo much cheaper.
Sometimes the items are allowed through without any extras - a busy day or a happy customs person, so you can be lucky and not have to pay the extra, but just don't bank on it unless the total cost is below the allowable limit.
It can also depend how the seller marks the item on the green customs declaration. If you're lucky they'll mark it as a gift and no customs to be paid, at least that's what I've found. Whether it would still be the same for an expensive item, I don't know.
I was told by a very experienced bee keeper not to get involved with the refractometers sold on ebay as they are only good for jam and if I wanted one for the honey then I will be expecting to pay £200+ as the tolerances of ripe honey and unripe honey are quite close so the tool has to be precise. I tend to leave it to the bees the best way to test the ones on ebay is test unsealed and then sealed honey sorry if I have upset people who have bought them from ebay but its what I was told