It can be changed even if you purchase eBay's shipping labels through eBay. I advise to use the cheaper of the two, Priority Flat Rate or regular Priority by weight, and refund the buyer the difference if it is a significant overcharge (usually anything greater than $1). It makes no difference in shipping time between the two. Be aware that some buyers will complain about the cost of the flat rate priority even though it is cheaper than Priority by weight. Thus, First Class and Parcel Post with delivery confirmation are also viable options to be considered, depending upon the weight/value/delivery need date/destination of the item.