One-time payments can be added when you first create a Rent Collection or afterward. See here for details on how to set up Rent Collection.
If you are creating a new Rent Collection, you can add one-time payments from the second step ‘Enter rent collection terms’. There, you can add a security deposit, last month’s rent prepayment, as well as any other one-time fees (e.g., move-in fees).
If you’d like to add additional one-time payments after a Rent Collection has been created, you can do so by editing an upcoming invoice. For details on adding one-time charges once the Rent Collection is set up, see here.