My thought is that it is $20 total. Seller has 100 percent feedback and I would think if he gouged on shipping that the rating would not be the case as others would feel the same.
I sell quite a lot on Ebay. As such I don't like the thought of being painted with the same brush as "cesspool for scum to rip you off". There are quite a few sellers on Ebay that are good, honest merchants that are just trying to sell off items or make a living. I myself have over 2400 positive feedbacks with many, many repeat customers. Only 1 negative feedback and that was from a chap in Ireland that I had to get into the NPB program to even get a response from. He paid late, I shipped right away and then he leaves a negative feedback saying that I did not communicate (sent him one invoice, three invoice reminders, two ebay system emails, the NPB contact, confirmation of payment received email, confirmation of item shipped email) and that he still did not have his item. Funnily, he negs me on the 16th of the month and USPS shows the item delivered on the 15th the day before he negs saying he hasn't got the item. Anyway, that side of things is a different story.
I count it no different than buying something at a swap meet, local store, mail order store etc. You have good and bad experiences depending on the timing, who you are dealing with and quality of product. I still feel Ebay is a good place to buy, but is like anywhere else, you have to ask questions, get a feel for the transaction and do the best you can. If in doubt ask!
As a seller I don't list shipping fees on my listings. I use USPS almost completely and used to list shipping with a small fee added to cover PayPal sucking out their cut and purchase of shipping materials. When someone purchased multiple items then the system would add all together and give a discounted combined ship for everything based on ship weight. Worked rather well. That is until the USPS changed their rates. Ebay may have changed since I tried right after the rate hike, but when I started listing with the new rates and using the weights etc. to combine ship items I ended up with not enough being charged. Weight was not the problem, but rather the box size needed to combine the items was not taken into account and the amount collected would calculate low and I'd end up being short on the ship fee. Just easier for me to delete shipping on the listings, put in the listing that I'm happy to combine ship and if you have any questions or need a ship quote to your area please email me and I'll be happy to quote.
Don't even get me started on the USPS International fees!! What a huge jump! You are forced to pretty much ship everything using First Class to get a decent rate and there they only say "varies" for estimated delivery so it is a gamble when it will get there. A 3 ounce parcel shipped by the new other choice of Priority Mail international is $16 just from here to Canada! Does not make sense on a $10, 3 ounce cable parcel and all!
I will say it seems that pretty much gone are the days of Ebay being filled with Joe Shmoes that are clearing out the house and garage. Sure they are still on there, but you find that the large majority seems to be companies selling instead. Not always at the cheapest price, but sometimes you get a deal when all is in.
There are those that take advantage of the shipping deal. Even in the Ebay seller community there is an uproar about some of these businesses that start a listing at .01 or .99 or buy it now for $1 etc. for a high ticket item and then charge $200 to ship or some such. The huge shipping is always listed in the auction so you know up front. Just a way to get around Ebay fees. Ebay does not charge fees on the Ebay side for what it costs for S&H. As such, the seller saves that money. Where Ebay gets their pound of flesh is on the PayPal payment where they charge the seller for the total dollar figure collected including shipping!
It is a strange thing, but you see loads of people who buy the 99 cent $200 ship example over a straight forward seller that puts in the item at $150 plus $40 shipping and gets dinged with Ebay and PayPal fees both ends. Even though the straight seller would come out cheaper it always seems like the 99 cent one comes out with more sales all things being equal. Guess it is the illusion of getting a deal???
Anyway, sorry for the sore spot rant. Best of luck with your item. I'd email the chap and say "Just to confirm the total with ship is $20? Just want to be sure before I send you payment." Should clear things up. Stay polite and even toned. If the seller does come back and say the ship is $20 then ask him to tell you how he comes to that amount, method of ship etc. Standard Priority Mail is $4.60 for a parcel that would carry that. On average most seem to charge $4.95-5.00. Even at $5.50 that is not too untoward if the seller has to purchase any materials, cover the cost of travel to the Post Office to ship (although ship and in some cases USPS pick up at the door is available), is covering PayPal fees etc. so as not to come out in the hole after the transaction.
Best of luck with it and hope all works out well.
Cheers