Patti,
If calculated shipping is charging your customers $2.45 that is the cost for a 1, 2 or 3 oz for the item/package weight 1st class either retail or commercial rates.
If you want the shipping to be more than $2.45 you need to put more than 3 oz for the weight.
4 oz to 8 oz first class is $2.60.
Then it goes up around 5 cents per oz until it gets to 16 oz.
Are you weighing your items WITH your mailer to get the total weight to put in the listing when you list an item?
If you are only putting the item weight in the listing and then adding a mailer to make the weight go over 3 oz that is why you're paying $2.60 for first class because it's putting the total weight over 3 oz.
Weight in listing needs to be total weight of everything.