It can take 4-5 months via sea mail from UK - have had many heavy items sent to me this way from sellers I regularly buy from.

Your seller should have contacted you first before sending via sea mail as you had agreed on airmail and were expecting them to do this.

I would still be contacting the seller advising non receipt of your items.

If you paid via paypal using a credit/debit card but have missed the cut-off time for a claim you may still be able to claim back though your credit/debit card provider for non receipt of goods.

Also find out if there is any paperwork you can fill out here for AP to do a search for it - may be floating around at a PO somewhere awaiting delivery/collection.