The tracking on eBay is a bit of a hit and miss affair with regards to updating on time. Sometimes it works as if there is nothing wrong with it at all, the next minute it's all over the place with tracking not accurately reflecting which stage of the delivery process the item is currently at, even when the item has been received by the buyer.
If I remember correctly I think it's possible to leave a follow-up comment on feedback left. If you go to your Feedback section and click on the Feedback Left For Others section, look for the User ID of the seller in question and click on the bit where it says "Follow-Up", on the right hand side of the screen. You may have to log back in again, but once you do you should be able to leave an additional comment. If you want to leave a feedback comment but didn't add any text the first time then you should be able to get around the problem by leaving feedback this way. However, if you leave it too long then you won't be able to do so, as you only get a certain amount of time to leave a feedback comment.
In the event that the seller is unfairly penalised by eBay if they erroneously conclude that the seller has not delivered the item by the estimated delivery date provided, then any proof that you did receive it before the delivery deadline, whether photographic or in writing, would increase the seller's chances of being able to get the decision overturned. By adding follow-up in the manner that I described in the previous paragraph you could, if applicable, add in something along the lines of "Item arrived earlier than expected, and before eBay's estimated date of delivery - very pleased with item" just to let the seller - and eBay - know that the item arrived on time and that you were happy with how the transaction went. That way eBay would have no justifiable reason to impose a penalty on the seller for failing to hit the estimated date of delivery target if he/she could prove that you recieved the item before the target date.
