Your 0% interest period runs to December 10. Technically, you are correct that you are no longer in a "grace period", and technically Discover is charging you interest on the balances, as we speak. Interest of $0.00 to be exact.
The course of action you want to take is to time your payoff so that the card goes completely to zero by about December 8. This includes not introducing new charges, or including recent charges in that Paid to Zero on or about December 8.
Then let the account sit until the statement prints. Check that to see if any residual interest appears. If you've paid to zero by December 8, the answer should be no residual interest. If there is residual interest, it will likely be a very small amount. Pay it immediately to squash it.
Once that statement prints, then you are off and running with a new grace period timing.
Once you don’t pay an entire statement balance by the payment due date you have lost the grace period. You begin to be charged interest on all balances from that day forward. Existing balances, and new charges. All of it. During a promo period the only difference is you are charged $0 of interest but you are still being charged interest.
In order to get the grace period back, you need to bring the actual card balance to zero. In some banks, you may have to do this twice, across Statement periods, due to trailing interest.
Bring the card balance to completely zero just before your promo APR expires, for best results. And that means no use of the card after that, until you are certain no trailing interest appears.
In order to get the grace period back, you need to bring the actual card balance to zero. In some banks, you may have to do this twice, across Statement periods, due to trailing interest.
Bring the card balance to completely zero just before your promo APR expires, for best results. And that means no use of the card after that, until you are certain no trailing interest appears.
@Remedios wrote:
OP, do you currently have 0% APR promo?
If you do, you are not accruing interest right now whether you paid in full or not.
Interest will be assessed on any charges made after promo expires, or if you do not PIF the amount on the last statement before promo expires.
This wording could be improved 
The interest begins being assessed on existing balances, as well as new charges, not just any new charges after the promo period.
And, in fact the OP is accruing interest, is being charged interest right now during the promo period. It is just at a 0% rate resulting in a $0 interest cost.
@NRB525 wrote:
@Remedios wrote:
OP, do you currently have 0% APR promo?
If you do, you are not accruing interest right now whether you paid in full or not.
Interest will be assessed on any charges made after promo expires, or if you do not PIF the amount on the last statement before promo expires.This wording could be improved
The interest begins being assessed on existing balances, as well as new charges, not just any new charges after the promo period.
And, in fact the OP is accruing interest, is being charged interest right now during the promo period. It is just at a 0% rate resulting in a $0 interest cost.
😒