I like the Maritz invoice system.
Like others, as soon as I am finished filing my reports, I submit the invoice. To me, its just the last step of reporting the shop.
My worksheet even has 3 spaces for the invoice information: Invoice Amount, Date of Invoice, and Invoice ID. Right next to my Invoice Amount space, is the amount spent for the shop. I keep this record for various reasons. But, most of all, its to remind me when I go over or under the shop's maximum allowable reimbursement. My monthly spreadsheet reflects the non-reimbursed amount.
BTW, after my invoice submission, I save the information of the invoice in a separate file.
But, the reason I like the Maritz system is that I am in complete control of my spending and reimbursements. I know what I should be paid and when, and I have a record to prove it.
Its not that they (Maritz) have ever disagreed with my invoices, its just that after having dealt with other MSCs who did not request an invoice, btw and who pay weeks and months later (often incorrect amounts), I am very careful of my small business spending and reimbursements.