Based on your description, I'm guessing that your buyer is a reseller and knows exactly how much more the upgraded shipping will cost. Understandably, she wants to pay as little as possible for as much as possible, but you aren't in this for charity and shouldn't lose money to accommodate a buyer. (JMHO)
Note that for seller protection, you aren't required to have s.c. unless the total payment is $750+ and I don't get the sense it is.
"Dear buyer,
Thank you for your purchase. Had you contacted me prior to purchase, I could have revised the listing to reflect the costs of shipping by express and signature but your having purchased first makes it impossible for me to adjust the shipping.
I can offer 2 options. Either I ship as described and paid in the listing or I can send you a Paypal invoice for the addition cost to send by EMS with signature. Note that it will cost an additional $x, not $3 as you incorrectly estimated.
Let me know if you're willing to pay the extra shipping cost or if you prefer I ship as advertised."
Thank you BeenBurned - you've nailed it.
Yes I think she's a reseller, which is fine, but her expensive shipping 'preferences' and measly offer of a contribution towards them after agreeing to terms by buying and paying have annoyed me. Item is well under $750.
Your note to buyer is excellent! I'm about to contact her and this will help a lot. Thanks again
