With the Brexit fee
, we're essentially paying for the next years membership even if we aren't opting to taking the place.
(This is analogous to taking a ski-trip with friends. 10 people all agree to pay £900 to go on a ski trip. At the last minute, one person drops out. But everything has already been paid for, so do the 9 remaining people pay £1000 each instead, or does the person that dropped out pay their share even though they can't go on it. In this case, we are agreeing to pay the £900 by leaving in this manner).
Staying attached to the EU's Customs Union allows us to enjoy some of the benefits of being in the EU that we are paying for.
This "untested" temporary customs union looks a lot like Turkey's option to me. Actually I think the BBC might be referring to the wrong thing when they say this is "untested".
https://infacts.org/briefings/turkish-option/
https://en.wikipedia.org/wiki/European_Union–Turkey_Customs_Union
Of course once this transition period is ended, we again have to talk about what the customs border will look like. The only sensible decision would be to have the customs union in the sea between Britain and Ireland. Politically that's probably not possible for the Tories at the moment