In order to cast a ballot on the 2017 Worldcon, you must:
- Be at least a Supporting Member of the 2015 Worldcon, and
- Pay the Advance Supporting Membership (ASM) fee for the 2017 Worldcon. This automatically makes you a Supporting member of the 2017 Worldcon no matter who wins the election.
Both conditions must be true. Yes, this means you have to pay two separate fees to vote on Site Selection. One of them is your WSFS membership for this year, and one of them is your WSFS membership for two years from now. WSFS rules require everyone voting on Site Selection to be a member of the current Worldcon and also to commit to being a member of the Worldcon two years from now.
If you only joined the current Worldcon to vote on the Hugo Awards and you don't care who wins the 2017 Worldcon and do not want to vote on the Site Selection for 2017, you only have to pay one fee: the supporting membership to the 2015 Worldcon. If you aren't interested in Site Selection, then why are your reading this far?
The money you pay for item 2 is held in trust by the 2015 Worldcon. They turn it over to the 2017 Worldcon no matter who wins. The 2017 Worldcon then gives you a Supporting membership.
Money you pay to a bid committee before the election is a donation to that bid committee that helps them run their campaign. (This donation is sometimes called a "pre-supporting membership." Despite this name, it has nothing to do with a Worldcon/WSFS Supporting Membership. It is a donation to a political campaign, similar to you donating money to someone running for office.) If that bid wins, the bid may give you some sort of perk related to the amount of money you donated, such as a discount off an attending membership in their convention. You need to talk to the individual bid committee about what they are offering. If the bid loses, you get that bid's thanks for having helped fund their campaign, and that's all. (Personally, I support all bids at some level.)
I think Site Selection nearly always sounds confusing to people first coming to the process. The process is primarily designed for the convenience of the people who vote every year. If you vote in every Site Selection election, then in any given year, you're only paying for the membership in the two-years-hence Worldcon because you paid for your membership to this year's Worldcon two years ago. If a site you plan to attend wins, you then immediately convert to an Attending membership (at the lowest possible rate); if you don't plan on attending the winning site, you don't convert, but you already have a Supporting membership in that convention, so you're all paid up and will be able to participate in the two-years-hence Worldcon as a Supporting member, including the Hugo voting that year and (upon payment of another ASM) the election of the Worldcon site two years after that.
ETA: Apparently at least one person thought that the two conditions were OR instead of AND, so I've clarified that you must be a member of both the current Worldcon and of the 2017 Worldcon by paying the ASM. It's both, not either.