Call or email FTDNA and let them know that you realize that you should have ordered a 25 marker test and they'll change their records. Assuming the kit is already mailed - simply mark out 12 markers and write in 25 markers and enclose a check for the difference when you return the sample
I wish that 37 markers was the default when anyone went to order and that the other choices could be found by digging.
Here is my standard advisory on test ordering at FTDNA:
Which Surname (yDNA) Test?
o If you are trying to match to a different surname without a paper trail - I always recommend 37 markers as a minimum. 67 will be better if the persons you are matching are also tested at 67.
o If you know the surname you should match, you can probably “get by” with 25 markers, but many folks want (and sometimes need) 37 or 67 to get the clearer insight.
o If you are only interested in your "deep ancestry" (haplotype) or in proving that you don't share a common ancestor with a specific family, 12 markers are adequate.
o The only time we see 12 markers being sufficient as a positive match is when the paper trail connection is solid.
o I also tell folks that if they are serious about their genealogy - that they will end up with at least 37 markers – and very probably wind up at 67.
You can go in steps, upgrading a bit at a time - or buy the markers all at once (which saves you waiting time – and a few $$ on 37or 67 markers). Prices (through a project):
12 markers $99
25 markers $148
37 markers $189
67 markers $269
There is a $2 s/h charge for domestic orders and a $4 charge for international orders. Upgrades (using the original samples) from one test level to the next are $49. (37 to 67 is a two step increase and is $99)