I have done both -- or is that "either"? Anyway, never had it kicked back. Always got paid the amount they said they would reimburse. The important thing to remember is when you are invoicing, if you had more than one purchase, scan both receipts onto the same page, because you can only submit ONE scan with each invoice. I have had LOTS of things kicked back by the editors, but never had an invoice questioned.
Also, I heartily recommend making a copy of the page that comes up after you hit "submit", the page that shows the rate, any incentive, and the purchase, the total you will be reimbursed for, because when you get your paper check, if I recall correctly, it shows only the job # & the pay, and it's up to you to double check
it against what you billed. I'm expecting 6 payments on my next check, and believe you me, I will check every dime.
I learned to copy that page the hard way!