Other frustrations of working with shopping engines include:
You can cap but not truly control spend.
A consistent delivery cannot be promised. Spikes will happen out of the blue and might burn through your budget overnight. You can try to regulate that in your insertion order (IO) if you can.
The IO process is archaic. It lasts forever, has little detail, and can go across clients if you're an agency.
Shopping engines have multiple budget limits, account reps, account preferences, systems, and so on. Managing billing and credit departments is a nightmare, too.
There's no consistency in the channel, no best practices across engines across the feeds. They have not standardized the feeds, as each engine maintains its secret sauce.
Some solutions for the shopping engines:
Develop a spend limit based on daily spend, similar to Google AdWords. A monthly credit limit that simply shuts down isn't a regulated budget.
Allow advertisers to have more influence with various methods of optimization.
Have a third party develop a system similar to Google's My Client Center (MCC) to control multiple shopping engines and multiple clients.
Allow budget limits, preferences, billing, systems, and so forth to be managed by this MCC-like interface as well.
Develop a standard format across shopping engines. Unused fields could simply be ignored by specific shopping engines.
If we had a wish list for shopping engines, stronger optimization opportunities would be right at the top. That would make this logical, effective consumer touch point more a part of an integrated plan to maximize revenue and less of a crapshoot.