I'd start even earlier. It would be a good idea to do a little buying on eBay to get a better understanding of the process from the point of view of your customer.
Don't buy a printer here (although you will be amazed how useful one is. I bought a combined printer/scanner for about $100 at Staples).but consider poly or bubble envelopes and a small metric digital scale.
However, of course you can handwrite your parcels. You can even use stamps for postage (I do-- and another thing you can buy on eBay is discount mint postage). Ask at the PO for air mail stickers, and their special Small Packet USA, ExpressPost, and Expedited Parcel labels.
In many cases the service you use (not Letter rate) will have a special label with a number on it and perhaps a bar code. These are useful in case of slow delivery.
BTW, you do know that your customer's payments will be held by Paypal for 21 days against your good performance, don't you? However, PP will allow you to print a shipping label from your buyer's payment so you can meet the seven day standard.
