I have two tools that I use.
First, a spreadsheet similar to what Flash described. In a column on the far right I keep track of my net earnings for that 2-month period.
Second, I use a budgeting program that is similar to Mint, called YNAB. I have a budget category called "Secret Shopping." Every mystery shopping expense, reimbursable or not, goes into this category, with a few exceptions which I'll get to in a second. If I go out to eat and spend $30 but will be reimbursed $25, then I just spent $5 in my Secret Shopping category.
The only expenses that I don't count as "secret shopping" expenses in my budget are those that I would pay for either way. For example, when I do a gas station shop with a required gas purchase, I usually get $10 worth of gas. I split the expense into two categories: $5 (the reimbursement amount) for secret shopping, and $5 for Transportation expenses. If I go to the grocery store with a $9 reimbursement, I do the same. For any restaurant expense or gizmo that I'm required to buy, I call it a Secret Shopping expense (not for tax purposes, but for my own personal tracking). Basically, anything where I might be tempted to spend money that I would not have spent if I wasn't working at that store, is a Secret Shopping expense in my budget.
When I get paid, I do not record the payment as income. Instead, I put it directly into my Secret Shopping category. (I'm not sure how Mint works, but in YNAB you can record money as Income or you can deposit it directly into a sub-category). That way I can keep track of my net income including any outstanding expenses which are waiting to be paid. For my first 6 months of shopping, this category was in the red at certain points in the month, and if it got too far below zero I stopped accepting shops that required an expenditure. Once I was staying above $500 I started taking money out of that category every month to pay myself. I keep a $500 balance in that category to cover shopping expenses. (At some point I'll need more than that, but it is plenty for the level I'm at now).
I hope that makes sense. YNAB has a slightly different budgeting philosophy than Mint does, but I think you can apply the same principles to both.
We are all here on earth to help others....What on earth the others are here for I don't know.
--W. H. Auden
Edited 1 time(s). Last edit at 12/23/2015 09:33PM by Alter_Ego.