I found with my guide, who was overall pleasant, I had to be very firm with the 90 days.
He kept offering throughout our call suggestions to pay it over less than 90 days, and I kept saying things like "let's do this over 90 days". My suspicion is they have to be able to say the only way the deal would get done is if they spread the payments over 90 days. He never offered the 90 days, I inquired to him about it, and really to be honest more told him that this is how I would like to pay than inquired.
After the purchase was complete, he congratulated me, and said I made a really smart decision spreading the payments out over 90 days. He also had a good enough relationship with his supervisor that he was able to do the purchase first, and then get the approval from the supervisor after the fact. I really got the sense they only do these 90 day payments if the customer makes it very clear that's what they need, and aren't willing to consider anything else.
When/if I purchase my next contract, I plan on saying I'll do it the same way we purchased last time.