"-ll" is the contraction for "will." "Will" (and "-ll") do refer to future time, and also perform other functions. "Will" has several uses, including these:
To make a promise: I'll call you every day, I promise.
To indicate intention:
When I have enough money, I'll ask her to marry me.
My grandfather will be 85 years old next week.
To indicate willingness:
He'll accept the job right away if he gets an offer.
To mention something that always happens and that is always expected to happen:
Items will remain in the shopping cart for 90 days. (This is the usual procedure.)
"That'll be 25 cents" could be part of a conditional sentence like
(d), to indicate willingness:
That'll be 25 cents (if you
choose to pay it, dear customer).
Phrasing the sentence this way softens
the fact that there is a requirement to pay, pretending to leave the
choice of whether or not to pay up
Or it could be that "will" is used here as it is in (e), with "will" indicating
that something always happens and is expected to happen all the time.
It's possible to say "That's 25 cents," but it is less gentle than "That
will be 25 cents."