Jerryy

The reason I heard that it's so cheap to ship from China is they currently enjoy a massive discount on global shipping rates, mostly due to the fact they are crazily still classed as an "emerging economy" or something like that, their RMB is famously under valued as well apparently.
 
For Chinese Airsoft products it's can be difficult to export from China due to the product restrictions.  Few Chinese brands are allowed to ship directly from mainland China, generally have to ship from Hong Kong.  All adds to the cost, so fewer wholesalers, some brands only allow you to purchase 20ft containers so a wholesaler will wait till all stock is gone before reordering.  
 
On the note about UK stores not using cheaper shipping.  I sent Large Letter products 1st class for 8 years or so for only £1.50, eventually I gave up since I was getting a large amount of customers saying their item didn't arrive since there's no "official" tracking.  What customers didn't know tracking did indeed exist but only if you speak to Royal Mail business support directly.  I just got fed up with all the hassle from it, an being open to fraud so I moved over to Tracked 24 which is double the price - sucks but has to be the way forward.  

Maybe a little on the tight side, hard to tell with a shorter burst but not too bad.

I'm no expert but it sounds good to me.👍

If your gun has a quick change system simply buy a new spring, somthing like a m100 or m90 and replace it, if you know the model and make of your rif do a quick google and find how to replace the spring with your specific gun, i reccomend ak2m4 but other sites are available

Easier way is to buy a new spring.  Cutting springs can work fine also but can lead to other issues. 
 
I made a handy chart on my website on the XT springs that should give you an idea of where to start.  Some AEGs come short stroked from new, some super short inner barrels.  Lot of variables which affect power.  

If your gun has a quick change system simply buy a new spring, somthing like a m100 or m90 and replace it, if you know the model and make of your rif do a quick google and find how to replace the spring with your specific gun, i reccomend ak2m4 but other sites are available

is there going to be some tomfoolery happening?

Seeing as the Scar has a standard V2 gearbox, I would go for the Perun Hybrid V2 rather than the AB++ this will give you a lot more features and reliability especially if you are running 12:1 gears (personally I would just stick to 13:1 and you can definitely find them cheaper than that anyway -check AK2M4.co.uk he is cheaper for the ZCI barrel you have listed as well). Your motor is a similar story, I would go with an ZCI, SHS or XT brand one for about £30, the one you have isn't that special and is overpriced IMO. If you want better performance and don't mind spending a bit more, go with a warhead brushless. Everything else looks fine, not a massive fan of SHS ali nozzles as I had my M14 one snap instantly when dry firing the gearbox without even having a hop unit on (looking at it afterwards it was so thin I can't believe how poor the design was and I had to switch back to stock).
 
I have several double bell P90s and honestly couldn't be more impressed with their stock internals, the stock pistons are some of the best pistons out there and their gears and shimming all seem fine from the factory. It is amazing how good they can be out of the box with only un upgraded hop rubber and a brushless motor without even touching the gearbox. I have been eyeing up the Scar for a while but haven't managed to justify pulling the trigger just yet.
 
just realised after writing that that you have a replacement piston in your list. If it has the black piston with full metal rack and the red aluminium piston head already in your Scar I would just use that. I would buy up these pistons if I could find a place to as I love them and haven't had any problems with them in anything. If you still want to change your piston then you will probably need a new piston head as the piston heads tend to be thread locked on so tightly that it is impossible to separate them from the piston themselves.

1st which brand of scar do you have there might be compatability issues. Next what do want the gun to actually do? Next don't do everything at once you need to know what mod actually works. I would start with just the hop rubber and nub. See what that does maybe Next get the barrel. Would then get the mosfet (maybe at same time). see if that combination gives you the results you want before spending more money. Also if you don't have one get a chrono. No point moding a gun only to be told you can't use it on the game day if it fails a chrono spot check.