The Accounts Receivable part of Lynx is an integrated accounting module that works in conjunction with the Invoicing (and the General Ledger if you choose to use it).
Included in Accounts Receivable is the ability to set up and maintain taxes and tax groups, to age accounts, provide a quick A/R balance, and handle period-end billing.
Invoicing is a sub-part of Accounts Receivable. This is where you enter the various types of billing transactions (such as over-the-counter invoices, delivery invoices, service invoices, and finance and rental charges) as well as entering payments received.
IMPORTANT: All invoicing functions (that is, any transaction involving money) are always done in batches. This might involve creating new batches or adding transactions to an existing batch (assuming it's not closed).