There are a number of ways to deal with offline/cash donations
There are three options:
For a single cash donation bank the donation into your Campaign Page by donating it on your own credit card. Keep the cash and make the donation in the donors name. If you use the donors email address they will even receive a receipt immediately. The amount will be reflected on your page like all donations.
For cash sourced from one or multiple people simply bank it in using your own credit card and write a message such as "cash donations collected at the BBQ". A receipt will be issued and the amount will be reflected on your page like all donations.
Pay the cash or cheque donation direct to the charity. To add the amount so that it shows on your Campaign Page simply login go to the campaign in myaccount and select "add offline donation". There is no commission charged on offline donations and no receipt will be issued