Inverted sentence/ reduced relative clause?

"Attached is the receipt" is using the so called "subject-verb" inversion where the subject and the main verb switch positions and the word order becomes "verb + subject", not "subject + verb". The linked Wikipedia article on Inversion (linquistics) has some examples.

If you don't use the inversion, the sentence becomes:

The receipt (subject) is (an auxiliary verb to make the passive voice sentence) attached (a main verb).

Note: Attached could be considered as an adjective, but still it is a subject-verb inversion.

When the verbs such as to attach, list, enclose are used in the passive voice, it is common to use the inverted sentence as in:

Enclosed is the list of participants to the conference.
Listed are the names of participants to the conference.

The reduced relative clause is a clause where a relative pronoun alone or a relative pronoun together with the verb "be" are omitted as they are not absolutely required. Some examples are listed here.