CRA recently released new information on their position regarding the retention period for church offering envelopes. CRA has indicated that starting in 2016 (and including church offering envelopes for the 2015 tax year), church offering envelopes (when used as a source document) will need to be kept for six years from the end of the tax year to which the envelope relates.  Previously these envelopes were only required to be kept for two years.  

More information on CRA's policies regarding books and records for Canadian registered charities can be found here.