You wrote: "I just got my receipt for a taxable donation I forgot about."
I assume you meant "tax-deductible donation" and not "taxable." Are you sure you can even claim it? You appear to have used Free Edition (is that right?), which does not have a Schedule A for itemized deductions on the Federal return. To claim a charitable donation on a Federal return requires Sched. A, which requires Deluxe or higher. AND one has to be able to itemize in the first place. Does the total of your itemizable deductions (including the charitable donation) exceed the standard deduction?
Or is this a state-specific charitable donation for some specific charity that is allowed on your state return? If so, we can tell you what you'll need to do, once you find out if the filed return was accepted or rejected.
