Good evening Martin and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Before we begin, if you get the chance, I would recommend upgrading your server to PHP 5.4+ (in fact, WordPress typically recommends 5.6+).
I am sharing this information as I have seen some odd behaviours with older versions of PHP.
The short answer is yes. You can give discounts based on volume.
This will happen directly at the WooCommerce level (as it handles the transactional part of things).
To achieve that, you will have to choose one of the many volume/bulk discount plugins out there (based on your needs).
Here are a few to get you started:
Let me know if that helps.
Have a great day!
Geoff B.