I once saw a certificate that cost 1500 coins. I believe this was to be able to dig up dirt mounds. I didn't have the funds at the time but i have had the funds since then, but the certificate has never come back as an option. How often should this come back and will it ever be an option to purchase from the store or maybe as an upgrade option? I find it annoying that I am unable to use this feature and curious what would be found in the mounds.

The flea market rotates every 6-7 hours. Hopefully the certificate you mentioned isn't showing up when you're asleep at night.