It also seems that on occasion larger orders will require authentication or approval from Steam, due to a spending cap placed on your account by Steam.
If you try to place a larger order, and are not successful, it possible you'll need to activate Steam Guard to enable larger purchases.
There are some instructions on doing that here:
https://support.steampowered.com/kb_...4020-ALZM-5519
If that doesn't fix things, you'll likely need to contact Steam support and provide them with some information about you and your account to get the limit removed:
https://help.steampowered.com/en