Mystery solved. Looked back at the transaction record a bit closer. Turns out that because I had bought some stuff from HoS previously I was enrolled in their new AIRGID customer credit account, kind of like what Scotweb has. When I made my order this time they automatically deducted my accumulated AIRGID (I assume this has some meaning in gaelic) for creating an account and for subscribing to their newsletter and that is how I got the extra $10 off the price.

I assume, however, that anybody who opens a customer account with them and subscribes to their newsletter would get the same credit of AIRGID, thereby giving them a roughly $10 discount off their next HoS order.

All in all still not a bad deal for a pair of ghillies.