Here are few suggestions for your shipping practices.
Buy supplies on eBay. Buy your shipping supplies on eBay, where hundreds of etailers are ready to cater to your most specific shipping needs — from vinyl record shipping mailers to forklifts. Prices are competitive, if not better than many box stores. Also keep in mind that if you do prefer the big box stores, many, like Office Depot, offer free shipping for orders over $50.
Get a label printer and good postage scale. For $200 to $300, you can buy a thermal label printer on eBay or other online retailers. If you ship more than 30 or 40 items a month, you will find this an invaluable time-saver because It gives you precut, ready-to-stick labels. As for postage scales, there are also plenty of excellent deals on eBay. Expect to pay $50 for $100 for a good digital scale.
Pack and weigh your item before you list it. We mentioned this before, but it bears repeating. Once you've photographed the item you should pack and weigh it. You don't have to seal it (and it might be wise not to explain why). With accurate weight and dimensions you can select the calculated shipping costs in the Sell Your Item form. Then, shipping costs are automatically calculated for the buyer based on zip code, saving you considerable email correspondence with potential buyers. You can also calculate shipping insurance rates for UPS and USPS shipments at eBay.
Use USPS Priority boxes. If you're shipping via USPS Priority Mail, there's no need to buy boxes — the USPS will provide them for free. (See below for a few examples.) At the USPS website, search for "priority boxes."
Example: USPS Priority Mail packaging