What kind of experienced seller ships a small item in an envelope with a stamp and charges over $3 for shipping? There is no tracking. Once a seller did this and the sharp edges of the keychain cut through the envelope. What kind of feedback would you leave regarding the shipping service I paid for but didn't receive? Would you warn the seller that she could get scammed?
What did the listing say for the shipping service? If she didn't specify (nor charge for) priority mail, even shipping with a stamp and charging $3 isn't (IMO) overly excessive for first class mail. Stupid? Absolutely. If you weren't honest, you could claim non-receipt and would win the case.Because it is against eBay rules for a seller to use a less expensive and less secure shipping service than the buyer paid for, I think a neutral feedback is justified.
I think it's appropriate to message the seller about your reason for the neutral feedback, and also about the danger of being scammed.
IMO, if the item arrived safely and undamaged, I think I'd leave positive (at best) feedback or no feedback but wouldn't rate the shipping cost.
I would send a message (not in feedback) that she should consider tracking for her items. It would only cost $2.61 and could save hassles in the future.