@Collections_For_30_Bucks - Do NOT pay it. Here is why and what to do instead:
First, send a debt validation letter to the collection agency within 30 days of their first written notice. Under the FDCPA (Fair Debt Collection Practices Act, 15 U.S.C. 1692g), they must cease collection activity until they validate the debt. If you cancelled properly, they will not be able to validate it.
Second, check your credit reports immediately. If the collection is not yet reported, great. If it is already on your report, dispute it with all three bureaus and include your cancellation documentation.
Third, under newer credit scoring models (FICO 9 and VantageScore 3.0+), paid collections still show on your report. So paying does not necessarily help your score. Removing the tradeline entirely through a successful dispute is the better outcome.
Fourth, regarding your mortgage: talk to your mortgage loan officer. They deal with collections disputes regularly and can often work with the underwriter to exclude disputed items, especially small ones with documentation of the dispute.
If the collection agency violates the FDCPA (continues collecting after you request validation, threatens you, calls at improper times, etc.), you may actually have a claim against them for statutory damages up to $1,000 plus attorney's fees.