My suggestion would be to explain to your customer the reason why you are requesting this change; use the requirements of the Food Safety/Quality Plan you may have written per guidance of GFSI and your company's Approved Supplier Program. It is always helpful to include any reference of regulatory information when requesting changes to a customer. Assure you focus on the benefits to the Customer(s) in requesting the change. Always begin your communication; I am requesting the following addition or change in your process based on ...... reference internal risk assessment scoring, a new policy release, or new regulatory release. End your communication " We appreciate the support of " Our Company's" food safety and quality programs.
If the Customer(s) response is still leaning towards not wanting to comply with your request; I would suggest you reach out to your Company's Senior Management group and inquire about offering the customer financial asst if the refute has to do with cost that will be incurred.
As mentioned prior you could write an exception based on a completed risk assessment showing there is no or very little risk to your company and the end consumer; however I do not recommend this route when it comes to tamper proof sealing without very strong evidence to support " no risk".
The last option ; which is highly not preferred or recommend is to inform your customer if the change is not made per your Company's Food Safety Program's Approved Supplier Program; and Food Defense-Food Fraud program requirements receipt of the goods would have to seize until a proven method could be verified.