It was something salisburync said about corporate A/P systems, etc. that caught my attention. I suspect that many new shoppers come from that world, where the organization they work(ed) for makes a direct deposit to their bank account, like clockwork, on a specific date--like the 1st and 15th of each month, in the accounts by either 12:00 noon or 5:00 PM.
Working and getting paid as a contractor is not like that. You are not protected by labor laws that cover employees, including very strict laws that govern when and how an employee is paid, and thus there are no severe penalties for not following those laws.
Most of the MS companies try to pay by deadlines they have quoted, but as smaller businesses their payments take the maze of a route compared to a large corporate or government employer. MS money goes from their client's bank to their bank, then from there to (possibly) a payroll company, then there may be two or more additional levels before reaching paypal or our banks. One or more of those steps may even involve snail mail--and we know what that's like these days. There can be delays and glitches at any step.
As a contractor, it might be useful to think in terms of payments due by the month when it comes to getting paid for services rendered. Not specific dates--even dates quoted by the MS company. As long as the payment arrives within the next month, it's okay. For a shop done on June 1, I expect to be paid in July--and I give the company until July 31 to make good before I look into it. That's some 60 days after the shop.
I keep track of expected payments by month. For the month of January I have listed in my book all the MS companies I did services for and how much is due (reimbursements and shop fees) from each, with payment expected in February. I check off as each payment arrives. If it's a partial payment I make note of it and how much is still due. 99% of the time pay arrives the month after the shop month, February in this example, as expected. The remaining 2% tend to be companies (like Ath Power or HS Brands) that reliably pay in 60 to 90 days and state their terms on their web sites.
Only once in my 15 years of mystery shopping has something really weird happened over payment dates. That was when Marketforce had a major computer glitch a couple of years ago. No malfeasance or wrong-doing--just a glitch. And even they paid in full, like all the others, in the month I expected.