Good podcast gear gives you broadcast-quality audio that lasts five to ten years, so it pays to buy right the first time. Skipping cheap USB mics means you won't have to replace your setup the second your show grows.
November sales are the ideal time to buy audio interfaces and XLR mics. Gear manufacturers usually refresh their lines in early spring, which makes last year's flagship gear a much better value during holiday sales than the newest shiny release.
Keep in mind that an intro window is a deadline, not free money. If you still have a balance after the 15 months end, regular interest rates will kick in on whatever is left over. Pay off the $45.33 each month to hit zero on time.