In the US university where I was a TA we were specifically instructed not to get anything worth more then $15 from a student while we are responsible for their grades. Otherwise the student or other students in the group can send a complaint to the dean and accuse you that your grading was biased. 

In other countries, where customs, policies and procedures are different, the situation can be different (e.g., in Russia it would likely be fine). But if you work in the USA - I advice against accepting that offer.