Shoulder is one of the most important fits to get right, most other measurements can be taken in/out a little bit. Tight shoulder will pull across the back and will make range of motion more difficult. For me, i wouldnt go more than 1/2" less than my ideal shoulder measurement but i do go up to 1" larger and then get the chest and waist taken in. The price seems good
Chester Barrie make (or made) good quality stuff, but speaking personally, I wouldn't get that jacket.
Firstly, the cut looks a bit dated, as the buttons are lower than is the current trend. This was a popular style back in the 1980s, but nowadays (and in earlier decades, too), the buttoning point is typically a bit higher.
Secondly, as the cut looks rather dated, it is too expensive even though I'm sure that it is well-made. You should be able to find a something such as a brand-new-with-tags (BNWT), made-in-Italy Polo RL navy jacket on eBay for only $100 or $200 more than that. Alternatively, here in Australia, look at Herringbone or MJ Bale during sale time if you happen to have a store near you.
Thirdly, the jacket has metal buttons. This is a stylistic choice - blazers usually have metal buttons and some people like them. I prefer brown horn or corozo nut buttons, as they look nice without standing out quite as much as metal buttons. Personally speaking, I think that metal buttons make the jacket a bit less versatile as I'd feel a bit odd wearing it with jeans as well as with trousers.
Fourth and finally, don't buy a jacket if it looks as though the shoulder width is too narrow for you as that is one thing that you cannot alter - you can let out quite a few other seams (by a little bit, at least) but you can't let out the shoulders of a jacket. Before buying online, it's best to find a jacket that fits you well and to take the measurements from that. You can then compare that to the stuff that is available online so as to get a better idea of what will fit you or not.
Its amazing how much knowledge you guys have. Thanks a lot.
Hopefully in the next couple years I will be in the same position spreading my knowledge to newer members