What to call a person or process of giving someone money to buy something for you if he/she is at the shops?

In English, what do you call a person or the process of asking someone who is on his/her way to the shop to buy you something for you or to give someone a parcel to give to someone else if he/she is going towards the direction of the intended recipient?

Examples:

Please buy me coffee on your way back.

If you are on your way home, please take Sams phone he left it behind

I am looking for a word to describe the process or person, similar to messenger

Thank you