How to get Participant Fee as Part of the Total Contribution, using Webform Integration
Step1 on the post you referenced will expose the Participant Fee field to the webform List item
Step2 sets the amount for it


The lineitems are automatic from there on. I have webform_civicrms that add on a Contribution (Donation amount) as well as a Membership!

[UPDATED] - posting some additional screenshots: I hope this will help you!


End Result:

The Start page looks like:

Note that the Membership and the Event Participant webform fields live on the Start page; the Donation one lives on the Payment page. 
Start page - webform Elements:


Membership -> I decided to present all live options - that means it will take the Membership Fee as specified in the CiviCRM Membership Type settings;


Event -> two pairs: _participant_event_id form key and _participant_fee_amount form key / one pair for each event - I've set these fields to hidden -> and I've specified a $ amount -> then when someone selects the event -> the fee is added automatically.