If you’re on Android this is already possible via IFTTT Pro. I posted instructions to Twitter but I don’t think I ever posted them here on the community so here they are.
First up make sure you have IFTTT installed on your phone, you can get it here: https://play.google.com/store/apps/details?id=com.ifttt.ifttt&hl=en_GB
Go to https://ifttt.com/create/
Then Android Device > Notification received from specific app.
App Name: Google Play Services
Filter by notification keyword: with amex
Next up select Google Sheets > Add row to spreadsheet.
Spreadsheet Name: AMEX Transactions
Formatted Row: {{ReceivedAt}} ||| {{AppName}} ||| {{NotificationTitle}} ||| =REGEXEXTRACT("{{NotificationMessage}}", “£(\d+.\d\d)”)
Then save.
Go back to https://ifttt.com/create/
Then Google Sheets > New row added to spreadsheet.
Find the AMEX Transactions sheet IFTTT just created by going to http://drive.google.com/ and paste in the spreadsheet URL.
Next select Monzo > Move money into a pot.
Pot: Select the pot you’d like money moved to.
Amount: {{ColumnD}}
Then hit save.
The final things to do are to make sure you get notifications from Google Pay. Go to Google Pay on your phone, select your American Express and make sure ‘Show all activity’ is ticked, then go to the app’s settings and make sure ‘Purchase notifications’ are switched on.
Then go to your Monzo app and make sure your Amex Direct Debit is set to come out of your designated American Express pot.
When you buy anything using your AMEX, even if it’s with the physical card rather than Google Pay the value will be added to the pot of your choice. The only thing to note is that you’ll need to make sure you dismiss each Google Pay notification. If you already have one notification and a second payment arrives, the second payment won’t be added to your pot.