She's definitely a full boxer though
Evidentially you have absolutely no proof of that. Did you buy from Pets4Homes? On the Key Advert Facts, is the "KC Registered" Box showing at all? And is it showing as Yes? If it isn't, mentioning "papers" in the advert is very ambiguous, especially in a court. It may be a case of Buyer Beware.
I've bought from Pets4Homes (a cocker spaniel) and am thankful I knew what I was doing because some of the tactics used to sell from dodgy breeders is dreadful. I would say 80-90% of the "cocker spaniel" litters (inverted comer deliberately used) may well not have been actual cockers.
There were clear cross-breeds being sold as cockers, with pedigree prices. There were ones that said "KC Registered Parents" - code for the puppies not being KC registered (you'd state this if it was the case), but that the breeder wanted to use the words "KC Registered" so that buyers don't realise what they are buying.
There were ones where the breeder had clicked "no" next to the "KC Registered" box to cover their back, but then in the description gone on to mention the puppies coming with "paperwork" (which could mean anything). Again selling at pedigree prices.
There are sellers registering as private (not business) and leaving the "KC Reg" box blank so that it doesn't show up on the advert. Then going to great lengths to mention heritage of the "pure breed" (another code word for not KC registered - because you'd say that if it was), but no where in the advert actually stating KC registration, just leaving it to be incorrectly assumed.
Even when you do all the right checks, make sure you have a KC registered breeder who is registered with ChampDogs and/or KC website - You can still get sellers outright lying. Like the seller I visited who didn't have parental KC documents even though the litter would, apparently, be KC registered (I don't believe they would have been).
Suffice to say OP that you need to be sure you were not duped into assuming the puppies would be KC registered, when in fact there is no written statement to prove this.