Hola,
I just spent 3 weeks in Guatemala, and I took the bus from Playa Del Carmen to Chetumal, and then took a bus (24 passenger minivan) direct to Flores, Guatemala.
John did a really great job of explaining the process--it is very easy and you should have no problems.
When I crossed from Mexico to Belize, I tried very diligently to not pay the $20US they were asking me for...I asked why, and said I don´t understand...(in spanish...lol) I stated I would not pay, and that there are no fees...the guard took my FMM card at that point and said OK...at that point I did not want to lose the FMM card, because that is what originally shows my stamp for 180 days....I bitterly gave in to the $20US...received my FMM card back, and then when I returned to Mexico this past Sunday, the guard stated that my passport was in order and they asked for no other fees and let me through.
I used San Juan Travel from Chetumal that goes direct to Flores via Belize City Water Taxi Station...this was the only option from the bus terminal...it costs 400MP to Flores, and I think 200MP to Belize City.
It is roughly 4.5hrs from Playa Del Carmen to Chetumal, and
roughly 4-5hours from Chetumal to Belize City depending on time crossing the border.
There are several busses that leave Playa Del Carmen for Chetumal throughout the day and late night. From Chetumal, there is a 7am bus that leaves Chetumal for Belize City and continuing on to Flores. **I believe there is only 1 bus leaving for Flores via Belize City.
There are two busses that leave Flores daily...7am and a 9:30am, and will take about 4 hours to get to Belize City depending on the time to cross the border.
I hope this helps!
snow