First, let's state some facts upfront.  FAR 5.201(b)(1) requires the government to make notices of proposed contract actions through the GPE at the threshold contained in FAR 5.101(a)(1), which is any proposed action expected to be over $25K.  We also know that we use FBO for the GPE.  FAR 5.202 list exceptions to the posting requirements required by FAR 5.201.

Your specific question is about FAR 5.202(a)(13) with states:

The proposed contract action--

(i) Is for an amount not expected to exceed the simplified acquisition threshold;

(ii) Will be made through a means that provides access to the notice of proposed contract action through the GPE; and

(iii) Permits the public to respond to the solicitation electronically;

  All of these actions have to be met to be an exception to posting.

(i) Is for an amount not expected to exceed the simplified acquisition threshold. 

• This one is self-explanatory, if it's over the SAT we can move to (ii).

(ii) Will be made through a means that provides access to the notice of proposed contract action through the GPE.

• You mentioned it seems clear that it must mean it's posted in FBO, that's true but not directly posted on there. This means that if you are using a system to transmit the notification and that system then interfaces with FBO (GPE) it meets (ii).  For example, the Navy has Navy e-commerce online (NECO) which interfaces with FBO. 

(iii) permits the public to respond to the solicitation electronically.

• Because this was predicated on (ii), if there was a system being used to interface with FBO then the contractor would need to be able to submit electronically.  It could be through the system used to interface, and I've found nothing that defines it in a way it couldn't be email.  The contracting officer would need to describe how they would like the contractor to respond electronically.