One of our clients, Veritas, has not paid our invoices for a long time even though we have asked ithem to pay several times.

Companies and organizations are usually referred to as they/them.

The total of overdue invoices is 56.250,00 YEN.

You can see the invoices in question, inas attachments.

Another way is: "Attached are the invoices in question."

Therefore, we request you tor assist onance on the collection of the invoices.

One of our clients, Veritas, has not paid our invoices for a long time even though we asked ithem to pay several times.

The total ofamount of the overdue invoices is 56.250,00 YEN.

You can see the invoices in question, in the attachments.

One of our clients, Veritas, has not paid our invoices for a long time even though we asked ithem to pay several times.

Companies are usually referred to in the plural, e.g:
"Have you heard from Google? No, I haven't heard from them"

The total of overdue invoices is 56.250,00 YEN.

You can see the invoices in question, in as attachments.

Therefore, we request you to assist onthat you assist with the collection of the invoices.

The original sentence is understandable but I think it sounds better with these changes :)