We've used Selling manager pro (SMP) for 5 years to manage virtually every aspect of our Ebay sales.

95% of sales are paid by Paypal and most are paid at time of sale.  SMP sorts sales into categories (eg sold-unpaid, sold-awaiting postage, sold-paid+sent)

When items are paid by Paypal this is recorded automatically in SMP - this tells us what to send - sold-awaiting postage.  We grab our trusty barcode scanner and our boxes of (the new style) prepaid sign or tracking labels from APost.  As we go through the sold+paid list we work out what needs a sig and what doesn't - expensive sig, cheap track.  Scan into notes the tracking number, update info as posted and date, then print off the invoice and packing slip (all option) from SMP. Before leaving a record paster tracking number into the record which is held in SMP in case needed.

Then move onto the next item until all done.

Then fold packing slips (modified inside SMP to function as legal tax invoices) to slip into a self adhesive invoice enclosed envelope.  Write on back the item being sent (easy if you have coded it).  Stick the relevant labels on the front to the window and when you go to warehouse/garage/spare room stick the envelope on the item.  take all the items to the post office and away they go.

items not paid by paypal - we check bank account on line every day.  Tick off direct payments into SMP which then go through the same procedure as above.  some payments come in as credit card transactions following calls or faxes - same process again.  NB We select the right type of payment mode so we can reconcile Paypal vs bank account transactions.

We've processed around 5,000 transactions this way, and with multiple locations and multiple users.  In five years there have been less than five stuff ups between paypal and SMP (payment made, but not recorded in SMP).  We chase buyers up if they haven't paid within a few days.  You can be sure to hear back from someone who has paid and you ask them to pay up.  Guaranteed!  Then you go through Paypal history and find the transaction.

A pretty cheap and seamless method to manage mayhem.