I am sorry you and your daughter had such poor customer service. The member of staff reminds me of two individuals.
I use to work with autistic adults who attended Special School. One was lucky enough to get work experience in a local independent book shop. ( Sadly no longer there)
He upset about few customers as he was very abrupt in how he spoke to customers.
He said something similar to another child.
He also, directed a woman to a diet book as she needed it. He had no idea he was being rude.
In my local co op there is a autistic man who fills shelves. People often interrupt him asking where a certain product can be located. He comes across as very abrupt.
He makes noises and stims.
Regular customers step in and help him with other customers.
I am not excusing the upset caused to you and your DD and better customer training is needed for the individual who spoke to your daughter.
I would email or write to the store and explain the upset and suggest better training.
Regarding the lanyard have you got the one that says 'carer' on front that may be more helpful. They also now send out badges too.
I have my own 'classically Autistic ' adult child and can relate to your upset. After 30 years DC has learnt now admits he learns in a different way. Although, I sometimes still have to speak up.
In this situation I would of said ' We likes these and dont need any help'.
Terminology and attitudes have changed. However, words and thoughtlessness still hurt.
I hope your DD continues to find bookshops and libraries as a 'happy place'.