I just did this, and it sucks, but it's the only way. Origin does not have a cart option, so it's three transactions. The support department was kind enough to give me discount codes on top of the existing discounts to offset tax.
1. Buy Mass Effect Trilogy.
2. Buy ME2 DLC bundle.
3. Buy ME3 DLC bundle.
I don't know why they aren't putting out a complete pack, and support says it isn't in the pipeline.
I don't know why Origin doesn't have a cart, but apparently that's "By design" circa 2015. No plans for a cart system.
They did have me go through verification on the account, and tried everything in their power to set up a cart, even on their end. And they couldn't, because apparently that isn't a desired outcome.
So + 5000 points to Support, for being allstars. and a Kick in the Quad for EA and Origin sales departments. Because seriously, it shouldn't be this difficult to buy something that came in a box on other platforms.
I'm not even mad that they're separate products, but you should NOT have to make 3 card transactions.
My bank did flag it, but they were cool, I just had to say it was legit.
Oh, Origin puts a warning on your account if you buy more than 2 products in a row. Gee, maybe a cart would fix that?