Hi, just like the other guys here I would send a reminder email. I also got the following email/message from one of the threads on this forum, so here's another example which you can use if you wanted to and alter to your liking. 

"Thank you for your purchase. Your item has been packed and is ready to be posted. Payment can be made via Direct Deposit, Paypal ....etc. Would you kindly make payment as soon as possible / OR / by..... add a specific  date."  Kind Regards.......

To add, I have found some buyers have been slow to pay lately, but I have found once I have sent a reminder, they have been pretty quick to respond, some apologising and paying quite quickly after that. I suppose sometimes people do get side tracked with their busy lives.

Good luck!