Tell the seller that his offer is not acceptable. Tell him you will be returning the item with Delivery Confirmation for a full refund.
He is not required to refund your return postage.
If more than 35 days have passed since the auction ended, open a Paypal Dispute for Not As Described.(Paypal site, upper right corner, Resolution Centre.)
If more than 45 days have passed, go to the credit card you back your PP account with and phone the customer service number for a chargeback. They will probably want you to return the item with Delivery Confirmation, although some cards will accept proof of shipping rather than proof of delivery.
Act promptly, since over a month has passed and you are close to deadlines.
As to feedback, slow delivery and a failure to explain your actual options, as well as an offer of replacement instead of refund, would lead me to leave less than positive feedback. And much less than the usual five stars for a pleasant transaction.
Detailed Seller Ratings are much more important than feedback. The lowest Rating is ONE star. If a seller's DSRs drop below 4.3 his activity on eBay is severely restricted.
Oh, you have 60 days to leave feedback. But get your money back first.