Zero credit exchanges are often held alongside other gift exchanges which cost credits. With a zero credit exchange, you can earn a credit through completion but it doesn't cost any credits to sign up for the exchange.
The recommended amount to spend of 20 dollars does not apply to zero credit exchanges. All you are required to do for the zero credit exchange is send your giftee an item related to the theme of the exchange.
Some examples of zero credit exchanges we have had in the past are Stickers, Holiday Cards, and Postcards.
Please note that for zero credit exchanges, we often have more people volunteer to be a rematcher than we end up needing, so there is the possibility that you will not be assigned a rematch giftee for those exchanges. However, we almost never have enough rematchers for the credited exchanges and could always use more!