We have the same use-case. Our company pro-rates usage based on number of days, and we use a custom UOM to achieve that. However in the invoice we cannot show the EndDate on the usage line item. We also need access to other fields that are available on UsageSummary such as EndDate, AccountName, ListPrice and TaxAmount. Given we are using Individual Rating Option feature, we should be able to get the TaxAmount and RatingAmount as well. I want to add that the reason why we can't usage UsageSummary is because we have a custom field on Usage record that we want to display with each Usage line item.
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