What is good practice for approving spends that meet the charity's aims?  (Apologies if the terminology is incorrect)

We are all volunteers and have committee meetings every 6 weeks or so and often the school asks us to fund something. The meetings never have all members present but are quorate and the discussion always lead to approval though there is never a vote. I've no issue with the mechanism.

Sometimes the Chair will email the committee members a funding request. It will be something like "The school would like some books, they'll cost around £600, please let me know if you have any objections." Whatever the item, it will be sensible and uncontentious and for an amount that doesn't really disturb the bank balance too much. However everyone in bcc'ed and there is no mechanism for open discussion since we don't officially know anyone else's email addresses. 

Where I am unsure is the difference in approval methods - the first with discussion and visible approval, the second with no group discussion and tacit approval. 

The latter doesn't sit well with me. In a large national charity I am active with, if something came up that needed a committee decision and we couldn't meet in time, our secretary or chair would declare a time bound email meeting (the process is covered in our rule book). It would be discussed and voted on as required. At the end of the time, the meeting is closed, and the result recorded.  

So my question is what is the standard practice for making asynchronous decisions within a small charity setting?