A solid storm door changes your entryway right away. You get less draft on cold mornings, better airflow when you want it, and a layer of protection for your main door against the elements. The install usually takes just a couple of hours, and early fall is the sweet spot to get it done before contractors get slammed with winter emergency calls.
We are looking at about $550 for a heavy-duty model with a screen. Before you buy anything, get at least two quotes from local handymen or installers so you know you are not overpaying for the labor.
Keep in mind that the intro window is a hard deadline, not free money. If you still have a balance left on the Citi Simplicity Credit Card when those 18 months are up, the deferred interest kicks in on the whole original amount, which wipes out the whole point of this plan.