I think the 29-30 week is a big ultrasound. They measure stuff, etc. I would ask for one. I mean if you are paying, what's the big deal?
I was only supposed to have 1 with each of my pregnancies too. I've had 2 this time because they did 1 to date, pending some medical emergency I won't have another.
I had my first (and supposed to be only) ultrasound with Lucas at 18 weeks. Then he failed a NST around 26 weeks so they did an ultrasound. Turns out he was breech, so they did another at 32 weeks to check him out. He was 98%tile at that point (but he had turned) so they did another at 36 weeks to determine if I'd need a c-section. My OB didn't think I needed a c-section, but induced at 39 weeks because she knew he was big. I ended up needing a c-section... he was a lb+ what she thought he was going to be and never made it past my pelvis.
My ultrasounds are $703 a piece. My insurance company requires a letter for every ultrasound over 1 and if they don't find it medically necessary, they don't pay. One of my ultrasounds for Lucas was denied (the one where they looked to see if he was breech, they said it was preventative and they don't pay for preventative testing). Luckily, I had 2 insurance companies at the time so when primary kicked it back as having no benefit secondary paid because they hadn't paid for one. I think insurance is the primary reason doctors are saying only 1 per pregnancy, but thats just my opinion.