How to Add product type donation into cart programmatically with custom amount(100 or 200 or something)?
No code needed, just watch the video about "Donations with Custom Line Item Types in Drupal Commerce". Some more details about it (= quote from that link):


  This screencast shows how to use a custom line item type to allow your customer to specify a user-defined amount to donate. The line item has a Donation Amount field on it, and we then create a product display node that uses that custom line item. Finally, we use a custom rule to update the "price" of the donation product to the amount the customer has chosen.