Django shopping cart/basket solution (or should I DIM)?
Since you asked: if your needs are that limited, it does sound like a DIY situation to me.  I don't see what's so fiddly about it; what complexity there is is all in the pricing formula, and you're planning to supply that either way.  Add in Django's built-in session support and you're most of the way there.
I'm about to build a site that has about half a dozen fairly similar products. They're all DVDs so they fit into a very "fixed" database very well. I was going to make a DVD model. Tag them up. All very simple. All very easy.
But we need to be able to sell them. The current site outsources the whole purchasing system but that's not going to fly on the new site. We want to integrate everything right up until the payment (for both UX reasons plus we get to customise the process a lot more).
The other problem with the outsourced problem is it doesn't account for people that don't need to pay VAT (sales tax) or for the fact you get a discounts if you buy more than one of the same thing, or more than one SKU at the same time.
So I've been looking around. 
Satchmo looks like a whole mini-framework. It has listing options that I just don't need with the quantities of SKUs I'm dealing with.
django-cart has been re-hashed as of March but it looks pretty abandoned since then.
I'm looking for something that will let me:

pass it a model instances, a price and a quantity
apply a quantities formula based on the number of unique SKUs and copies in the same title
list what's in the cart on every page

That's about it (but it's quite fiddly, nevertheless). I can handle the final order processing nonsense.

Or am I just being silly? 
Should I just get on and Do It Myself? If that's your vote, I've never built a cart before so are there any considerations that are not obvious to somebody who has only used shopping carts before?