shipping quotes and flat rate shipping

i am having a real problem selling items bcoz i am taking too long to come back with a shipping quote as my nearest Fed-ex or Post office is quite a drive and pretty much out of the way.

so would it be fair to offer item for sale with a flat rate for shipping?

im thinking of listing everything from now on with a shipping included price ...so that way there is no waiting for quote and save me time and trips to the terminal it will depart from.

my only concern is there will be times the buyer under pays and there will be times the buyer over pays with a flat system......so if the buyer over pays .........and is sore about it ...then what .....can a buyer really complain if its a flat price that is the same for everyone in the lower 48......or might this be a potentail issue?