Hi @JHebert,
Thank you for reaching out to the Community!
So you created an invoice in HubSpot, right?
Or did you use another tool (like this one for example: “Invoice Template Generator”)?
If you are using the invoice in HubSpot, I checked for you, and at the moment you cannot customize the template for invoices.
I would highly recommend you to please post this product suggestion on our ideas forum here.
I’d recommend supporting the Idea with an upvote and explaining your use case in the comments.
Our product team uses that forum to collect ideas on how we can improve the product for our customers.
Product pays particular attention to the top voted ideas & takes them into consideration along with other feedback channels as well as their own vision to understand what features will be most valuable.
I also wanted to invite a couple of subject matter experts to this conversation @Shadab_Khan, @ChrisoKlepke and @Alex_Elborn do you have a workaround to help @JHebert, please?
If anybody else has anything to add and/or share, please feel free to join in the conversation 
Thanks and have a lovely day!
Best,
Bérangère