Balance transfers at 0.0% Promotional APR for n months are lucrative for card companies in various ways. One Discover offer charges a transfer fee of the larger of $10 or 5% of the amount transferred, with 0.0% APR for 12 months. Thus, if you take this offer and pay off $1000 on your other card,
your transfer balance at Discover will be $1050. Furthermore, as long as
any part of this transfer balance is not paid off, any purchase
made on the card bear interest from the day the charge is posted till the
day it is paid off; there is no grace period of 25 days from the end of the
billing cycle to pay off that month's bill in full with no interest being
charged. For those used
to paying off credit cards in full every month, this can come as a rude shock:
to those who are used to carrying a balance, it is just part of how the world
works. Your stratagem of not charging anything on the card at all is
a good idea, but even here, note that if the minimum payment required is,
say, 10% of the outstanding balance and that is exactly what you pay
each month, you will still owe $296.55 at the end of 12 months, from which
point on you will be charged 1.25% per month, same as for purchases.
In short, paying off the balance in full by the end of the 12 month
period is a better idea than carrying this forward.
You might also want to calculate what APR you actually paid if
you paid 10% of the outstanding balance for 11 months and a lump sum
for the rest at the end of the 12th (for a total of $1050) and compare
that to the touted 0.0% rate.