The final version of the code has yet to be published, but the institute has already expressed reservations about the way some enclosures are used to make people feel guilty.
Research published in November last year by the Fundraising Standards Board found that only 16 per cent of respondents felt it was reasonable to include gifts in mailings and most people said they would not give a donation to a charity that had sent them a gift.
But some direct mail specialists maintain that what people say and what they do are not always the same thing and that appropriate use of enclosures offers a good return on investment. So what can charities do to make sure they use enclosures successfully?
1. Link it to your message Donors are more likely to welcome an enclosure that specifically links to a charity's work, rather than an arbitrary gift. A famous Help the Aged campaign, for example, sent out pieces of clouded plastic in a mailing to raise funds for cataract operations. It was intended that people would hold the plastic up to their eyes to simulate what a cataract does to someone's vision.
More recently, the Children's Society included a toothbrush in one of its mailings. "It allowed us, quickly and tangibly, to demonstrate our work with young runaways alongside the strapline 'No time to pack when running from abuse'," says Jemma Wilson, direct marketing manager at the charity.
Stephen Pidgeon, a direct mail specialist who chaired the institute's committee that wrote the draft of the new code of practice, says: "Even a pen can be relevant - the best mailings refer to it asking the punter to use it to do whatever the charity needs, or they simply say 'we're enclosing a pen for your use so you can keep our details close at hand'. Umbrellas are not relevant, and lots of other stuff sent in the mail has no relevance to the message."
2. Don't just try to make people feel guilty Enclosures such as coins that are used purely to make people feel guilty are disliked by donors.
"Our research showed that 70 per cent of the respondents felt that charities put gifts in mailings to make people feel guilty about getting something for nothing," says Jon Scourse, chief executive of the FRSB. A further 93 per cent said that money spent on gifts might be better spent on the cause.
Pidgeon says: "There is absolutely no justification for a charity to send out coins. They are exploiting the good instincts of their supporters. It harms the reputation of all charities in the UK."
3. Test it out Consider testing whether a mailing with enclosures actually produces a better return than one without an enclosure. Wilson says: "We tested the pack without the toothbrush and the response dropped by more than half, which clearly showed that the toothbrush did work and enabled us to raise more funds and add value to our message."
4. Find out what people think One of the conclusions of the FRSB research was that charities needed to have a good relationship with donors. "The overriding challenge for fundraisers, whatever method they use, is to put the donor at the heart of the giving relationship," says Scourse. "Our research showed very strongly that the public wants some say in how charities contact them."
5. Make sure it's recyclable The recycling issue is increasingly important politically, says Pidgeon, and some law-makers could try to use the green agenda to kill off what they see as junk mail. It is very important for all enclosures sent by charities to be recyclable from now on, he says.
An FRSB spokeswoman says: "People polled are quite firm about their concerns regarding the environmental issues around direct mail - not just enclosures, but mailings in general."
Wilson adds: "In the toothbrush pack, we encouraged people to return the toothbrush so that it could be used again in the future. This enabled us to cut costs because we could use them for future mailings instead of buying more."