Categories organize the line items used on Flute invoices and catalogs.

Categories are a supporting structure for the invoicing catalog rather than a required one.
Line items may be assigned to a category.
A category can also be deleted without deleting the line items that reference it.
Those items simply remain in an empty category rather than being removed.

Each category returned by the API also reports an item count.
This gives a quick view of how many line items currently belong to it.
With that information, a merchant decides to rename, reassign, or delete it.

Each category has:

* A name, which must be unique within a merchant account
* An optional default unit type that applies automatically to line items created under it.


The Categories endpoints support listing, creating, updating, and removing categories.

* GET /pay-int-api/categories.
The list endpoint accepts a search string along with paging and sort parameters.
A merchant catalog with many categories can be filtered to relevant results rather than paged through in full.
* POST /pay-int-api/categories
* PUT /pay-int-api/categories/{categoryId}
* DELETE /pay-int-api/categories/{categoryId}