Donations Page with recurring and one time
I'm not sure about the integration of this with Webforms (I assume you can do it), but if you can use a regular CiviCRM contribution page, you'll be able to set up a page that offers both recurring donations and one time donations.

Under Contributions>Manage Contribution Pages>"Your contribution page">Amounts tab

You will see a field to define fixed donation amounts:


You will also see a field to define whether you'd like people to define their own donation amounts:


The third field you want to pay attention to is for defining recurring contributions as you'd like to. This will put a checkbox where a user can decide whether they want to make a recurring donation or a single donation:


Put it all together... and your CiviCRM contribution page will look something like: