If it says member, just joining starts as a member, so join, which is free. Which is pretty cheap.
However reading their website small sports equipment which excludes skis can be part of the baggage allowance if you are a Prefer Member (which is free). The only advantage seems to be that weight or size (of the various "small" items) is not an issue for members. This likely is a benefit mainly for golfers.
The large items are clearly separate , cost $150, have nothing to do with Prefer Member status and one pair of skis is allowed per fee. They are not part of the regular allowance though so that means normal luggage is not reduced by the paid for items. But there is a size and weight limit, with very specific limits for surfboards one presumes due to how large they are. Will they look for how many skis there are? Probably not unless the bag is obviously carrying a lot of skis.
I can imagine depending on where you board they may not have seen a lot of skis so the size of the bag may not signal an abundance of skis over the quota.
On the other hand $150 a bag to them is better than $150 for two crammed into one, so expect to pay $300. After a peek inside.
It seems a little unfair that a huge surfboard which must be bigger and heavier than 4 skis is treated the same but airlines are not renowned for their logic.