Depends on whether you're willing to risk losing $5
If you do just the Seal Brown 'At' test, it will tell if there are 0, 1, or 2 copies of At present. If the result is 0 or 2, you have a clear result. 0 would mean your horse is definitely not brown, and 2 would mean your horse is definitely brown. When you have only 1 copy present, that's when the test for recessive non-agouti 'a' comes in.
Brown 'At' is recessive to classic bay 'A', so if you have a horse with only one 'At' gene, and the other gene is 'A' your horse is At/A, and visually a classic bay. However, if the other gene is the recessive 'a' then your horse is At/a and is brown. (Technically there's a possibility that the other gene is wild bay, 'A+', but there's no genetic test for that right now and it is assumed to be quite rare).
So, if you do the Seal Brown test first for $40, and it comes back with a result of 0 or 2 'At' genes present, the mystery is solved, you know what your horse is, and you have saved $20-25. However, you can still get the more ambiguous result of 1, and you'd have to send in another sample and request the Recessive Non-Agouti test for $25. There's only a $5 difference in price between ordering the tests separately versus ordering them together, so it really boils down to how lucky you feel