You can sum cells across multiple sheets in Excel. This article presents steps for Adding single-cell values or Adding values in the range of cells.

So this is how you can enable multiple selections in a drop-down in Google Sheets. While this is not available as an in-built feature, you can easily do this with some Google Apps Script magic.

This is the demonstration file to accompany the article, How to display multiple grand total rows in a Microsoft Excel PivotTable by Susan Harkins.