How can i ask my mom for a small favour?

i like to write fanmail letters to celebrities and more time then not we will go to the post office together and buy american stamps for return postage anyway how could i ask her to buy some postage accesories like irc prepaid envelopes american stamps. i know it will be a bit more expensive than my usaul trip to the post office should i offer to use the money my grandmother gave me to pay. even possibly uk stamps thou i wont be sending to him now anyhelp would be great guys