@kyos, no problem at all!
If you are asking if the form stripe plan_id's that are used to populate the select box are stored locally on your app the answer is no, they are received from stripe.
You could save them if you want, take a look at webhooks, using them you can be informed if a new plan is created an you could store them in the database.
An other option might be that you cache the value received from stripe containing the subscription ID's (Cashier might do this already, I don't know).
There is no danger in retrieving the plan_id's directly from Stripe, this is only a lot slower then retrieving them from your own Database. And if you are afraid that someone might change the post data you can always check (using webhooks) if the details of the payment send to stripe match your own database.
And the select box should be populated with the plan_id, and this ID is send to stripe, not the amount someone has to pay to you, so they can't 'manipulate' the form (easy) that they pay 5 dollar for the 50 dollar plan.
Update: There are a couple of lessons on Laracast about Stripe, even if you will use Cashier, watch all those lessons, it will give you a much better idea on how stripe works: https://laracasts.com/search?q=stripe