Depends what their current terms are. Years ago you used to have to scan/submit copies of paper bills and they'd be very confusing about which month to submit (for example month 7 being your 8th bill which could be your 7th if any was pro-rata'ed at the start), then it went on to digital submissions with specific dates which were a lot easier. Since then I've tried to avoid them, especially when the RPI increase in costs is on the whole contract amount and not taking into account any 3rd party discounts.