Should I declare one or two tables to represent Items and Sub Items?
This looks like a straightforward one-to-many relationship, which as you suggested you can model by setting a foreign key in the child items table referring to the parent items table. As for performance, you won't have any kind of performance hit at all for a small site. Modelling your schema properly is your priority.

Consider renaming the tables too. For example, if the shop offers a cleaning service then you could think of the tables in terms of Services (e.g. car) and Costs (seats, trunk, etc)