Payment link in Civi invoice
You can use &quot;Default invoice payment page&quot; setting which is used to set contribution page that can be used to pay pending or partially paid contribution using payment processor.
More about the setting and uses:
Wiki: https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=346750980
Here is how you can achieve this

Create a Contribution Page by enabling Other amount and payment processor.
Navigate to Administer &gt;&gt; CiviContribute &gt;&gt; CiviContribute Component Settings and select contribution page create at #1 for Default invoice payment page
Create a contribution for a contact with pending status.
On User Dashboard of contact (/civicrm/user?cid=[contact_id]) under contribution section you will see pay now button against the pending contribution. When clicked on pay now it will redirect to online contribution page where one can pay pending contribution online using credit card.

You can also generate the link to pay pending contribution online http://domainname.com/civicrm/contribute/transact?reset=1&amp;id=[contribution_page_id]&amp;ccid=[contribution_id]

where contribution_page_id(id) is contribution page id created at #1 and contribution_id(ccid) is contribution id created at #3.
6. You can use this link to send in invoice for people to pay online.
HTH
Pradeep