Reinforcing this helpful excerpt from @Boardspace on Twitter:

Do not make the property manager require board approval for the purchase of low-value services or products.  If the property manager cannot be trusted to spend a few thousand dollars on the condo’s behalf, then the board has engaged the wrong manager. A limit is necessary, of course, and set in the contract with the property management company. Typically the amount is between $1,000 – $3,000.

It is the responsibility of the property manager to provide advice, share expertise on the best information possible on three quotes on a job along with recommendations, but it is the sole responsibility of the Board to make the final decision.