To use Garmin Pay you need to add your card to Garmin wallet:

• Download the Garmin Connect mobile app on your phone.
• Open the app, go to Garmin Devices, choose your smart watch and select Add Device.
• Accept Garmin Pay’s Terms of Use.
• In the app, select Garmin Pay and then select Create Your Wallet.
• Create a 4‑digit password.
• Select the card type Visa.
• Enter the required card details, such as card number, expiration date and CVV code.
• Accept Luminor Terms and Conditions.
• Get a one-time verification code via SMS
• Enter the verification code in the Garmin Connect app.

For paying you only must access the control menu of the watch by pressing the button to select Garmin Wallet. A 4-digit code (which you have chosen) must be entered and then you can put the watch on the payment terminal.

About Garmin Pay read more here.

To pay with Fitbit the card must be added to Google Wallet

• Download Fitbit mobile app on your phone.
• Open the app, find your smartwatch and click on Set up a Device. Create a 4-digit password. 
• Find you smartwatch on Fitbit app, click on it and select Google Wallet. 
• Click on +Add to Watch button. 
• Choose the payment card to be added to the smartwatch. 
• Follow the on-screen instructions.
• To set up Google Pay or add new card to your Google Wallet, follow the instructions here.

For payment press and hold the action button or menu on the watch and tap the wallet icon.

Select the card you want to pay with and hold the watch over the card reader in store.

To open a wallet on your watch, the 4‑digit password you created during the wallet setup process will be required once per 24 hours or each time you put the watch back on your wrist or turn the heart rate monitoring back on after it has been switched off.

About paying with your Fitbit you can read more here.