Sometimes when you have sold an item abroad to an Overseas buyer, they may find the Shipping costs a bit high and ask for Surface Mail instead.
It is not a good idea to send by Surface Mail. Surface Mail tends to have a rougher transit than normal Air Mail and often a package can take over 8 weeks to arrive. Even if you advise a Buyer of this, it is very likely that you'll receive an email in a few weeks time enquiring where their item is, have you sent it yet? or can you put a Trace on it? The unhelpful answers to these questions are "I don't know where it is because Surface Mail is untrackable", "Yes, it's been sent" and "No."
If a Buyer was to grow impatient or the package was delayed (which often happens with Surface Mail), they can put in an "Item Not Received" Dispute with PayPal. They have 45 days in which to do this (Very rarely will a Surface Mail Item arrive within this time frame). As there is no Tracking Number you can supply to PayPal, the Buyer will win the Dispute and have all their money automatically refunded.
Probably a few weeks later, the Buyer will then recieve the package. Will they let you know and repay? Possibly, but unlikely.
Remember, state all Shipping costs upfront on your listing to avoid any surprises and do not offer Surface Mail.
TIP: Sometimes it is cheaper to send multiple orders as two separate packages.