Some e-commerce sites ask you to add your items to a basket, like you might in a bricks-and-mortar shop. Some suggest you pop them in your cart, like in a 'real life' supermarket... in America. (We have trolleys over here, as everyone knows.)
However, try slipping something in your bag in a real shop - either side of the Atlantic - and you'll soon find yourself in bother. Just ask Winona Ryder. It's the internationally-recognised sign for "I'm not gonna pay for this, it's mine now."
Yet somehow the wording 'Add to Bag' is gaining popularity with e-commerce sites.
Of course, trying to map online activities to offline ones is a dangerous quest, as there are often no direct equivalents. With most online retailing - music (ha!http://www.message.co.uk/ I wish!), books, electronics, fashion - the offline versions rarely use baskets or carts/trollies. You just sort of wander around holding stuff until you're ready to go to the till.
That doesn't make for a compelling online call-to-action: 'Kinda hold on to this for a bit, yeah?' Hmm, I doubt our clients would go for that. The alternatives (add to basket or cart) might not be ideal, but at least they reflect legal activity!
I'm sure somewhere I've seen the wire container-agnostic phrase 'add to selection', which I rather like. It's does reflect a real-world situation - what you have is a selection of items. Best of all, it avoids any clunky mis-matched metaphors from the physical world of grocery shopping. (Of course, if you're building a new site for Tesco then basket (for a quick shop) or cart/trolly (for a big shop) still makes sense. But never 'bag'.
Ditch the baggage
You might say "if we're stuck with an imperfect metaphor anyway why not just go with 'Add to bag' and be done with it?"
Well, I would prefer things to be more usable, not less so. I would wish all users to understand the experience without any tiny moments of doubt.
But most of all, we would rather be making the web a better place than simply accepting a trend for the worse.