Hi,
It is not possible to club 25 workorders into a PR. even if you club them, they will on different lines and you can proceed with invoicing after each of the SES
The solution would be to define services of 25 units in the workorder itself. Thus one cleaning service would mean 25 wells. This will create only one service item "cleaning 25 wells". after this is done, only one SES will get created and then only one invoice. I dont think there is any way of restricting invoices based on price in std SAP
Else you may need to go for some custom coding
Regards
Vivek