underpaid shipping amount

Kia Ora b,

This is very frustrating when this happens,

Have you checked the Buyer's address to confirm if it is Rural or Residential? 

If it is rural you can place feedback stating you are awaiting rural shipping cost and you have sent them an email and to please make contact with you - ask them to check their spam/junk email folders incase your email has gone there - Otherwise please provide you with a Residential address so you can promptly send the item.

Some people use a different email address for their TM purchases which may not be checked through the weekend that's just been - I hope your Buyer is well and you hear asap - Cheers.

Yes this a common occurrence. Usually done in error but not always. Just send an email thanking them for the purchace and noting they made an error in paying urban when rural. So then first suggest that they supply an alternate urban address (many more receptive to that as not demanding $s), and if not possible let them know the extra rural amount required and the account to be paid into