Fortgoofy, Welcome to the DIS boards!
Yes, what you've said is correct, credit card payment can only be made for the full amount due, or you can write a check. The other option is monthly debits, for which there is no additonal charge.
DIS-NH Dues bills are generally mailed out after the annual meetings, as they have to be "officially approved" by the voting body of directors. And usually arrive the later part of December.