It depends what stuff you need as to whether it can even be shipped USPS. You're not going to find a reputable company that will ship hazardous materials like fragrances, essential oils and lye via USPS. Offhand, I know that for non hazardous materials, Dianna's Sundries and BrambleBerry both have USPS options.
Depending on where you live. I live in Michigan and if I order from Nature's Garden (Ohio) in the morning I generally get it the next day or if they are busy the following day. They are really quick.