First time adding postage to a label. It charged me but now I can't get it to print. This was on my phone.. I tried using another device ( my tablet) but now it wants to charge me again. How can I get it to print without charging me again? 

From the help center:

What if I print with postage and then can't or don't send the book, or my printer malfunctions?

• If you download a wrapper with postage and then your printer malfunctions, remember that you can reprint the wrapper (you can download it again) using the RePrint Wrapper button without being charged.  This can be done from any computer that has a working printer attached.

Can I reprint the wrapper?  Will I be charged again?

• Yes, you can reprint the wrapper.  No, you won't be charged again.
• To reprint, just click Print Wrapper (or RePrint Wrapper, if you have marked the book mailed already) on the request, and reprint from the Wrapper Settings page that comes up.
• Of course, each wrapper can be used only once, to send one shipment. (Printable postage, like postage stamps, can't be re-used.)

Hope you can get your printer to work.