They now digitally print stamps at the counter. So, I believe they can actually print you off stamps of any denomination. So they should be able to just print off 30 x 13c stamps from the machine. (To bring the 55c stamps up to 68c).
I was in a similar situation on Friday where I only had 60c stamps. Instead of giving me proper 8c stamps, that have a picture of a swan on them, that you have to lick and stick. They just printed off a bunch of 8c, peel and stick, type stamps with a generic Xmas image on them from the machine.
As a matter of interest they sell small denomination lick and stick stamps in values 1,2,3,4,5,8,10,15,20,25 and 50.