Late Fees can be calculated automatically by the system. These can be added by clicking on Settings (the cog/gear icon in the upper right) and then Fee Structures:

To edit an existing fee structure click the edit icon next to the fee structure. To add a new fee structure click the + in the top left:

Give the structure a name and then you can add fees. A structure could have multiple fees (for example if you have a different charge for different items, or different amounts at different time intervals, etc). Click the + to the left of the fee structure to add fees.

In the new fee item popup window you set different variables for when and how fees are calculated by the system. The method can be a flat $ amount, a percentage or some combination of the two. If you want to set a max amount that fees add up to you can enter that or leave that field empty. If there is a minimum outstanding amount before you charge a fee (say a tenant only owes you $75 on the rent and you don't want to charge a late fee for anything less than $100 you set the min. fee outstanding balance amount to $100).

You can set the income item (Late Fee) and the item filter method allows you to choose what the fees can apply to:

For example if you only charged late fees for outstanding rent you would select 'Include items' and then check the box next to Rent. Click Add and the fee will be added to the structure. You can add more fees to the structure but keep it mind all the fees will be charged. To have different fees (for different buildings, etc) you will need to make separate structures. Be sure to hit the save button once you have added fees!

To add the late fee structure to a lease profile (to make it a default for a building), see the Adding, Editing, and Deleting Lease Profiles article.To apply the fee to the tenant simply go to their lease info under Accounting -> Tenants or on the Properties page Lease tab. Click Edit in the bottom right and change the Fee Structure field.