Prince Harry and Meghan Markle's charitable org has hit a snag -- being declared delinquent by the state of California, but there's a perfectly good explanation ... snail mail.

Here's the deal ... the Archewell Foundation was found to be "delinquent" this week on paying its renewal fees -- and it's technically barred from fundraising until that gets fixed. On its face, it sounded like the charity wasn't taking care of business ... but that's not quite case.

We're told the reason Archewell was able to hit this not-so-great status is because a check to pay for these registration fees was simply never received, although we hear it was sent.

In other words ... the money to re-up its certification as a nonprofit just got lost in the mail -- and now, we're told another check is being cut ... and this one should cover the fees owed.

We're sure there'll be additional correspondence sent over to CA officials -- as the AG's original letter to them made it pretty clear that they ran the risk of being in bad standing if they didn't get it together ... that, or be booted from the Registry in one fell swoop.

The letter from the AG notifying Archewell of its delinquent status also made clear that if there was a delay in getting the fees paid, there better be a good explanation to make up for it.

Of course, Harry and Meghan are very much still involved in charity and philanthropy work -- as we just saw them in Nigeria for an unofficial visit ... where they made the rounds, attended events and chatted with the locals.