For anyone that cares...
I had enough and after the seller didn't honor their March 20 offer of a refund and difficulty getting a response from her, I called Paypal and they filed a claim and issued me a refund - 180 day protection. I offered to provide the tracking number and the messages I sent and received from the seller on eBay, but was told it wasn't necessary. I feel bad about it and will send the payment again if the plastic sprues show up, but TBF, the seller's tepid reaction and offering nothing but an apology and no response to me asking her to file a claim with Royal Mail, just caused me anxiety - all over a $36.96 transaction.