when you look at the list of school apps on LSACD, some of them have a little credit card logo, like a mastercard and visa, i think. that means that you have to pay with credit card for those schools. for some other schools, you have the option of whether to pay by credit card or by check. for the rest of the schools, you have to pay by check.
i applied to 13 schools total, and had several of each. wherever possible, i paid by credit card.
in place of a signature, some schools allow you to certify your app electronically. i found that the schools that would not take credit card at all were also the schools that wouldn't do e-certification. so you're not mailing just one thing; you're mailing two: certification letter (signed) and check (signed). you only see the certification letter after submitting your app, so don't worry about that yet.