If all donors opt out of including this suggested donation, a project can be fully funded if only the subtotal is given. If some donors opt in and others out, a project can be fully funded with only part of this fee allotted. You can find out how much money your project needs to be fully funded without the suggested donation by entering the amount left "to go" in the Give box and pressing "check out". Then click on either "Edit" or "suggested donation" and change the suggested donation to 0%. You'll then see the amount left to go adjust accordingly.
As an example, consider a project that has an upfront price tag of $400, which includes $60 to support DonorsChoose and $340 for materials/fulfillment. If a donor comes along and donates $100 toward the project, opting out of the 15% allocation, that would result in all $100 of their donation being applied to the $340 materials cost, leaving $240 of materials to be funded.
At that point, the 15% allocation gets recalculated off the ($240) materials that still need to be funded, such that the new upfront price tag is $240 / .85 = $282. Thus, after a donation of $100, the upfront price tag goes down not by $100 but by $118 - from $400 to $282. And of course, the total possible support to DonorsChoose from this project then goes from $60 to $42. And we won’t see any of that $42 if subsequent donors opt out of the allocation to support DonorsChoose as well. So, in this scenario the project would be fully funded when it receives just $340 in donations if every donor opted out of the 15% overhead/outreach allocation.