Strange ... I've purchased of HK site in US$ ... Euro's .. Polish Zloty's .... and UK Pounds ...
Always found that Currency I chose was what transaction went through in. I do not understand HK's " oh no its USD all the time" except that when payment is made to their account - it is converted to $ .... can't remember if its US$ or HK$ ... I seem to remember its US$
There should not be any excessive charge for this as the exchange is built into the pricing per currency. I notice that you seem to have a curious way to select and pay ... First I log in and make sure that I have Country and Currency showing correct top right of page. If I am travelling - it often reads the IP address and puts the country / currency where I am at that time. I never select items for Cart before changing these to my 'home country / currency'. (I have heard odd comment that leaving selection of currency to the later Cart stage can have problems). Then when I select items to Cart - I have correct currency and delivery automatically applied. Using Paypal - it then goes through correctly. I do check my statement at times and all show correct transactions with just slight difference for currency change from my Home Account. First thing I would check is the actual order that went through HK and compare to Visa ... then contact Visa as well to put your case. HK have very little to do in this unless the order supplied falls short against the price paid. Do not also forget that Visa / Bank Account will be in a currency you use and any online transaction may not be applied in same - so a small exchange charge is made.
I'll fly anything if I can launch it ! Youtube : solentlifeuk