Interestingly, both of those addresses should bounce without the box number. But it's a private mailbox service so they'll probably know he's #420.
Looks like the proper format is either 4lines. Name, PMB #, Street Address and Suite. City/State/Zip. Or alternatively: Street Suite 123 PMB 456.
I don't think his provided "Suite 108420" is correct but the USPS address validator seems happy with it. It does kick out the "South Twain".
Sorry, grew up with family who worked for USPS. End sperg... for now.