How do I update the Opportunity 'Amount' field with a formula that sums custom Opportunity Product fields?

The TotalPrice (UnitPrice X Quantity) across all Opportunity line items rolls up to the Opportunity Amount. If you wanted to affect the TotalPrice on the OLI, you could update the UnitPrice with the optional extras so that they are factored into the TotalPrice. This would then roll up to the Opportunity Amount.

In summary, custom fields cannot be made to roll up directly, however you can use them to affect the UnitPrice which multiplied by Quantity rolls up.

Hopefully that makes sense.