I use USPS extensively. They are the fastest and cheapest way to gat products to your customers. Priority mail usually takes between 2-4 business days. According to the weight, you may want to use the flat rate boxes. You can put as much as 70 pound in them and the fee remains the same. Light items 13ozs or less, use first class.
Since you are running a free shipping promotion, I suggest you use media mail, depending on the weight of course. I had a free shipping promotion for 20+ digital cameras and I used media mail. It only cost $3.22 inclusive of delivery confirmation for every one, to any destination in the US. That worked out well because the customers had to pay an insurance fee of $2.05, which of course i use as part of their shipping fee. Finally, using online postage saves you time and money Hope this helps.