Dallas here from the Shopify Social Care Team. I'm more than happy to help clarify for you.
You're correct when you say that all other payment gateways will require a transaction fee. The amount of what the fee is, however, will depend on what plan you're on. This is the link for all of the Shopify plans with all of their features and prices. (http://bit.ly/2NIVCSQ)
The section that you're going to want to look at is "_Additional fees using all payment providers other than Shopify Payments_." This will give you all the percentages for each plan.
If you're on the basic plan you're going to be looking at 2%, but if you're on one of our other plans you'll be looking at less.
All the fees related to 2Checkout are their own fees. If they have a fee per transaction, if they have a fee per month, or if they have fees in general to use their platform then you're going to need to pay those too in addition to Shopify.
May I ask where you're located? If you're in a country that supports Shopify payments is there a reason that you're not looking to use it?