Leaving aside any discussion of whether it is or is not ethical, I would have appreciated an initial mention that it was an affiliate link so that I could make sure to use it.
I did a bit of messing around after discovering that the postage on 3 kits to Australia would double the cost - using separate browsers to check the costs of one and two kits. Ordering the kits separately can get standard shipping on each, which is cheaper than 3 kits in the one order, which offers only "express" shipping (for a fairly loose definition of express).
In the end I don't know if I finished up using the affiliate link or not, because I was not alerted to it.