During the checkout process does the Apple Pay page offer a place to enter the shipping address or is the information prefilled and you have an option to edit the information?
Your iPhone may share with the merchant a limited set of your personal information, name, address, email and phone number. The information sent is contained at this path on your iPhone,
iPhone > Settings > Wallet & Apple Pay > Apple Pay Defaults
Only that limited set of personal information is shared and only if shipping is selected as delivery method. In store transactions shares only the name on the payment method.
The information contained on your iPhone and shared, may be used by the merchant to determine fraud risk. The information may be shared with your bank, third party verification services and internal merchant records.
You’ll need to contact the merchant to determine where the error is occurring. The merchant could be having POS or network issues. The merchant is your contact point for resolving the issue.