I agree. You need chairs. Like Alyssa said, the ceremony might be 15 minutes, but your guests will arrive early, you have to figure in the time it takes the processional, etc...
You don't have to get expensive chairs. Usually the wooden folding chairs are about $5 each. If you want to cover them, it is another $5 for the cover and sash.
If you want the Chivari chairs, those are $10-15 each, but should include the basic white pads. Custom color chair pads are an additional $5-10.
Does your count of 30 include the bridal party? There were 30 of us in Jamaica, but 8 of us were standing at the altar. We only set up 24 chiars.