Fixed Fee Billing with Work Items

I’m curious how everyone is billing a fixed fee while using various Work Items? It seems it would be best to be able to have one fixed fee for the Client and then add the various work items under that umbrella versus having to edit each individual work item’s fee to equal the fixed fee each month. We have ad-hoc projects throughout the year that are included in the fixed fee and work items vary each month, but having different work items each month skews the total amount and we end up editing each client’s amounts a lot. (i.e. Worker’s Comp audit, 1099s, etc.) Does anyone have any tips or tricks they’re using when billing fixed fee for multiple work items every month that doesn’t involve editing each work item fee for each client every billing or using one huge work item encompassing the full scope of work?