I'm not sure I follow your logic of not booking ancillary products during original booking. For me, it's quite simple as follows:

1 Add discounted ancillary when booking, and then:
1.1 if you change dates, you will pay the difference to the post-booking rate
1.2 if you don't change dates, you will enjoy the discounted rate

2 Don't add discounted ancillary when booking, and then:
2.1 if you change dates, you will pay the full post-booking rate
2.2 if you don't change dates, you will still pay the full post-booking rates

So for me, it's clear that you should add the ancillary products that you expect to use, given that 1.1 = 2.1 (in total charges) and 1.2 < 2.2, especially if the extra on 1.1 is recoverable from small claims court.