Can't speak to your exact situation, but it might be worthwhile to look into whether employment with the postbac institution carries tuition benefits. Personally, my job allowed me to pay a vastly reduced tuition rate while doing a DIY postbac. It's definitely possible to work a full time job while attending evening classes, but I found it pretty difficult, your mileage may vary. Just make sure you start off slow with a few classes to make sure the workload is reasonable for you.
If you get a tuition discount at your alma mater, as you say, that sounds like the best place to begin looking.
Hope that helps! 🙂
EDIT: If you need as many credits as possible, full time postbac makes more sense imo. If you are able to take classes AND work full time, that would probably be my recommendation if you can handle the workload (strategic class picking really helps here). Bonus: in my experience, adcoms look quite favorably on students who are able to work, take a busy course schedule, and still perform well academically.