There are serveral methods that may work:
The base template for the mails is in /addons/mail/data/mail_data.xml
There, I deleted:
"<div t-if="is_online and not record._context.get('proforma')" style="margin: 32px 0px 32px 0px; text-align: center;">
<a t-att-href="access_url"
style="background-color: #875A7B; padding: 8px 16px 8px 16px; text-decoration: none; color: #fff; border-radius: 5px; font-size:13px;">
<t t-esc="access_name"/>
</a>
</div>
<div style="margin: 0px; padding: 0px; font-size:13px;">
Best regards,
</div>"
(starting at line number 259; in template "Mail: Pay Now mail notification template")
Then, you have to 'upgrade' the mail module (goto apps -> search for mail -> three dots -> select 'Upgrade').
In settings->user interface->views, search for "Mail: Pay Now mail notification template"
Find and add "false and " like I write below in "bold" :
< t t-out="message.body"/ >< div t-if="false and is_online and not record._context.get('proforma')" style="margin: 32px 0px 32px 0px; text-align: center;" >
So if you need to cancel your choice, you only have to delete "false and "