4. ⁑Invoices
There are a number of bundles for typesetting invoices on CTAN (see the invoice topic) including the invoice package and the isodoc class. These are both available on MiKTeX and TeX Live and have English documentation available via texdoc.
The isodoc class has already been introduced in §3.4 Writing a Letter Using the isodoc Class as it can be used to create letters, so §4.1 Writing an Invoice Using the isodoc Class describes how to use that class to create an invoice. However, it may be that you need to add an invoice to an existing document or need to use a particular document class, so §4.2 Writing an Invoice Using the invoice Package describes how to use the invoice package. Additionally, the invoice package converts currency (for foreign expenses) and computes your totals for you, whereas with isodoc you have to do the calculations yourself.
Finally, in case neither suit your requirements, §4.3 Building Your Own Invoice using longtable and datatool describes how to create your own custom invoice using the longtable and datatool packages.
This book is also available as A4 PDF or 12.8cm x 9.6cm PDF or paperback (ISBN 978-1-909440-07-4).