Tyvek envelopes are labeled "Priority Shipping" which is meant for shipments over a pound. Anything under 16 ounces or under can go the cheaper USPS First Class rates in your own non-USPS envelopes & boxes, but the post office won't tell you that because they hope you pay for the more expensive Priority Mail, Priority Mail Flat Rate Small, Medium (two types with different dimensions listed by size only after the sale in the "Pay Shipping" screen) & likewise two large sizes. USPS will even mail you free Priority Mail, Priority Mail Flat Rate Small, Medium, Large, Regional A, Regional B boxes to encourage you to use their services.
Tyvek envelopes are usually the cheapest USPS method using their own labeled material, just protect what you can fit inside. However, their "Parcel Select Ground" may be cheapest using generic boxes & envelopes. Both eBay & Pirate Ships both contract with USPS, but eBay also has UPS & Fed-Ex discount agreements. The eBay rates are competitive with Pirate Ships.com, but eBay limits your choice to a three-dimension length X width X height box, even though they say "box or thick envelope", & even if you ship it in a two-dimension length X width envelope. Pirate Ships is less ambiguous about envelope shipping & have a dropdown choice specifically for it. Remember that envelopes can be cheaper because, though they still have a third dimension of height, you can fudge it. This is how Pirate Ships is usually cheaper, though eBay occasionally beats them. Just don't wrap it too tight in the envelope like a present or go too high because USPS will tell you to pay the box rate or return your package for insufficient payment. My post office guy is great & I think he lets me get by with a bit if fudge factoring.
Also remember boxes can be as cheap as envelopes if their weight fall within a shipping rate range within that of an envelope. Boxes weigh a bit more, but are protective. Generic boxes are always best because companies known exactly what size/weight combinations are the least expensive for mass shipping & save them the most money. The post office gives away boxes because they know exactly which overall will make them the most money.
