I just want to inquire about a situation where I was doing some volunteer work for a person by making some flyers and printing them , then she would pay me the money that I spent at the printshop and she gave me about a dollar more than the amount that I payed .I took it hesitatingly from her,as she told me not to worry about it .Is this considered a form of riba because I took more than the amount I payed ? Should I give this dollar for charity? Thank you.

Alhamdu-li-llah wa as-Salatu wa as-Salamu ‘ala Rasool-il-llah,
No, this is money that she willingly permitted you to have. The Prophet PBUH said,
“لا يحل مال مسلم إلا بطيبة من نفسه”
“A Muslim’s wealth is not permissible to others unless he gives it willingly”
Riba is exchanging money for money with an increase or the exchange of certain usurious items. Your case has nothing to do with Riba.
Allah knows best.