I am selling a small product and my commercial USPS rate is $4.68 for First Class Package (1-3 days delivery). It's a good rate, but the problem is I need a rate that is slower, to offer it as a free shipping option. Parcel Select Ground is $8.15 and takes 2-8 days, so it makes no sense for me to pay this as a slow and free delivery. First Class Package rate is in fact too fast, and I could offer it as a paid shipment. What should I offer as a free shipping option?