Seller forgot part of order but plans to send out rest

[thefrump53]

I made a purchase for an item Lego set which was sold as complete and showed so in pictures. I received my package March 1st and was disappointed to find a whole section of the set missing, I contacted the seller and they said it was a big accident and that they’d send what was forgotten out as an add on to the sale. This is fine with me, but I want to protect myself, how long should I allow for the seller to ship the second item? I want to ensure I can get a refund if they don’t send the rest of it within the eBay money back guarantee. 

[disneyshopper]

ebay's Money Back Guarantee expires 30 days after receipt of the item, so do not let that date pass.  If the date approaches and the seller still has not sent the missing pieces, go ahead and file a Returns Request from your Purchase history for Item Not as Described.