Display/Update Opportunity Amount from Products, Quotes, or Other Custom Objects

Problem

Your org has a complex quoting or opportunity/project accounting business processes and the Opportunity Amount field is updated manually. 

Solution

There are multiple solution designs, but generally for a quote rollup, do the following:

1. Create a currency formula field on the Quote that has the total value of the quote specified only for the records you wish to rollup (source field).
2. Sum this value up to the Opportunity object's Amount field (target field).