Depending on the luggage storage service provider you choose, payment is either handled before you book or after you collect your bags. We would always go with payment after you collect your bags as you never know what will happen throughout your travels.
Some service providers will charge you for a full day of storage up front, others are hourly with an affordable daily cap. For all services that we cover you would normally see a per bag pricing which would make the price calculation fairly simple if you knowhow many bags you are storing.
All online luggage storage services do provide an insurance on your bag while it is stored. Some of them even use a security seal, bag tag or similar to seal of the zippers of your bag. This is to ensure that no-one can access your bag without breaking the seal/tag.
If you choose an online service to find luggage storage for your bag(s), you can book in advance to ensure space for your bags. Most of the platform will ask you for the number of bags you wish to store, the drop-off date, and collection date.
A lot of main train stations do not provide luggage lockers due to security reasons. You might be able to store your bags in a locker if you hold a train ticket or similar.