From Spiffy Stores Knowledge Base
There are a number of issues that store owners have experienced when using PayPal Website Payments Standard to accept payments in their store. The most common of these is abandoned orders.
Orders are sometimes abandoned when a customer gets confused by Paypal's payment screens and doesn't complete the order. This can also occur when there are issues with PayPal's notification systems, or website. You can identify these orders by their status of "Payment Pending" when the payment method selected is "PayPal Website Payments Standard".
To ensure that there hasn't been a glitch somewhere along the line, you should also log in to your PayPal account and check to see if there is a payment there.
If there isn't a payment showing up in your PayPal account, unfortunately, there is nothing we can do about these orders other than to recommend a course of action on your part.
We recommend you follow-up with your customer by sending them an email asking if they would like to continue with the order. You would generally do this about 24 hours after the order has been placed.
If they do want to continue with the order, log in to your store admin, and view the order details. You will find that there should be an option to send order retry link, which will send them a link so they can complete their payment. When you receive payment, the order should then be automatically marked as paid on the "View order" screen in your Spiffy Store. If it's not and you have definitely received payment, you should be able to mark it as paid.
NOTE: The "Mark as paid" button appears on the "View order" page 24 hours after the order has been placed.