To clarify how I would interpret what Ceasesmith said ...
" the reimbursement is for a required purchase, and does not result in taxable income. In fact, if the reimbursement is $100 and you spend $125, you have $25 in deductible expense."
The key phrase here is "required purchase". So if your required purchase at, let's say a restaurant, and is any one entree and a drink with reimbursement up to $25. You decide that rather than buying the chicken and a soda which would cost $24.95 within the reimbursement you want the steak with bacon wrapped potato entree with a cocktail. The cost for that is $50. You cannot deduct the difference of $25 as you could have stayed within the reimbursement. But if you get to the restaurant and find they no longer carry that $24,95 meal and now the least expensive meal with a drink is $40. Yes , as you have no opportunity to stay within the reimbursement amount , you can deduct the difference of $15 as an expense.
This $600 reporting rule is not new, however third parties payment systems such as paypal have only been reporting earnings over $600 to the IRS for a few years. This reporting rule does not mean YOU do not have to report earnings under $600. It simply means the company you received the payments from only needs to send you and the IRS a 1099 if you earned at least $600. YOU are required to report all earnings to the IRS even if you did not receive a 1099.