Shipping sold goods from temporary mailing address?

[paragesb]

HI all,

I sell sports cards and I am working away from home, Hawaii....I am currently in Bremerton, WA.  I took up some cards to sell, to past the time.  I purchase and print postage with ebay.  I cannot find a way to change the Sender Address, I charge a flat rate, no matter what the cost would be, higher or lower.. I would think it would be cheaper or same cost to ship goods.  Am i able to send these pre paid postage labels at the local USPS?