Some parts will fit first class. I build a box and then put it in a generic padded envelope if it’s to heavy for first class, I then go flat rate padded envelope.
i believe my lower shipping cost compared to others gets me more sales and let’s me charge a little more for the item. YMMV!
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Please don't confuse ebay "asking" prices with "selling" prices. Many sellers never get their ask price. some are far from it. Value is determined once an item actually SELLS. Its easy enough to check SOLD prices.