@EvilClosetMonkey and @Tohster are of course right: vendors don’t want their customers be in full control and awareness of their spending at all times, because it keeps the users shopping and overall buying, which increases the shop owner’s revenue.
Despite such obscurity, there are other techniques to motivate customers to put more items into their shopping bags or carts. Some of these are well compatible with showing the current subtotal and may (over-)compensate the negative effects. This includes the following, without getting too creative:
- “Subtotal 15.90 € – Free shipping from 20 €”
- “Subtotal 86.50 $ – FREE gift for all purchases over 100 $. TODAY ONLY”
- “Subtotal 22.00 £ – 4 items in your bag qualify for our 5 for 25 £ sale – add one more and save 2.50 £”
- “Subtotal 98.60 € – All purchases above 100 € qualify for double Bonus Miles”
- “Subtotal 183.10 $ – Become a Gold Member of our Shopping Club with a single purchase of 200 $”
- “Subtotal 86.46 £ – For any purchase over 100 £ This Shop, Inc. donates a kitten to hungry children in Siberia”
That’s more marketing than UX, though. For a good customer experience, the current subtotal should always be available, but it should not be visible in your face, because part of the fun of shopping (at least for some people) is not to know or care how much you’re gonna spend. That’s assuming B2C, in B2B and C2C scenarios other factors play a role, e.g. I’ve seen a preference in office suppliers to offer candies etc. as incentives because that works better than discounts with the orderers (who are not spending their own money).