i apologize in advance if i sound dumb im so tired but can’t make my mind up

I sell custom canvas paintings & i feel like i get taken advantage of because i sell my paintings cheap compared to what others price theirs as. Im currently only selling in my small town and sometimes the people are unable to pick up when they say they can so i tell them i MAY be able to drop off for example my last order, the girl couldn’t pick it up on the day she said she could, so i told her i can MAYBE drop it off because i don’t drive yet so i had to wait for my partner to come from work, she ended up getting frustrated with me and came that same day to pick up her painting and gave me the money and left so quick! she ended up paying $10 less than what i priced the painting at. so now idk what to do if im not able to drop off i dont want no body getting mad because im the one painting for them! i just need advice, thank you in advance!