This places a considerable strain on the charity and its ability to deliver. Sadly, some small charities will enter all too eagerly into such arrangements in order to secure funding. The message is - don't. Take time when committing the organisation to any contract, especially a SLA which will contain very specific criteria for establishing and monitoring standards and performance by the charity for the duration of the contract. In the private sector you'll be used to dealing with consumers or end users.
Charities refer to their beneficiaries.
In your dealings with government departments you'll hear lots of references to service users. When all's said and done, it amounts to the same thing - the customer. It is acknowledged that many customers of charity services often have no choice. They either have to accept what's on offer or go without as the charity is frequently the only source of supply for that particular product or service. This means that product quality can become almost secondary to the ability of the charity to supply it.