SmithMicro and CP have had limited download windows for large files for quite some time.
It's very clearly stated that this is the case at time of ordering.

You get offered this "Download Protection" and a "Backup CD" at time of purchase, so you probably removed one or other of these options from your cart when buying.

All that the message you received is doing is offering you another chance to obtain an extended download window, just like a lot of retailers try to sell you an extended guarentee around the time the original is coming to an end.