Google my business
When did you last request the pin? If it's been more than 14 days, you should contact Google My Business support, explain that you've been having trouble getting the postcard, and they'll guide you through an alternate verification route.
You can contact Google support through the following:
Twitter support is normally better, but they've been slammed lately, and you should be able to get things squared up with the phone help in 'talk to a specialist'. Good luck.