Yeah, thats pretty annoying, postage is a massive killer at times, its definately influenced me at points. I'm quite lucky in that my (very local) FLGS has a generally very good range, and at least for warhammer carries decent discount and a loyalty scheme, although they dont have EVERYTHING in stock all the time, and there are a few others I visit periodically whilst passing which also have good discount, and mean I don't NEED to mail order most of the time.
The biggest issue I've found, even with TRYING to keep track using a list builder, is doing this over a fair amount of time, its easy to forget exactly where I was up to/what I've done, especially if Ive had to stop part way through a squad/squads and what I need to do next lol
The combat patrol stuff hasn't helped as I cant remember whether I've added it all to my list or not, although I've definately added a few bits; and the planned Dark Angel upgrade sprue parts to add to the mix, and conversions add to that. (Planning to use leftover DWK heads and terminator power fists and storm bolters on two copies of the Combat Patrol/Leviathan Captain for example to give a more Dark Angel Knight themed Captain, x2). All the Deathwing Terminators squad sergeants get upgraded DA Power Swords for example as well, and I cant remember 100%, but I've got a stinking feeling I grabbed some extras to build a normal squad too, that I could relocate the more puny standard Terminator Sergeant to haha
I've heard the Brutalis Dread thats in the Combat Patrol premium kit is a bit of a mare to build too.
Its one thing to remember it all when it was initially planned out. Not so easy 6-12+ months later plus a couple of second hand purchases giving rogue model numbers etc

I will definately have to do a recheck of my army list once everything is done incase I've missed anything.
Did pick up a load of magnets and magnetic sheets to go inside plastic crates a month or two back though, and a load of super glue gel to go with it a few weeks ago though, so the plan is that once stuff is fully built at the least, I'll go through and mass base magnetise and then it can easily be stored together which will help with preventing stuff getting mixed up later on, and make it easier to transport. Realistically, although not costing nothing, a hell of a lot more affordable than some of the alternatives for an army of this size, too.