We can customise the sales forms in QuickBooks Online but it has limitations. We can't add a stamp nor signature to it. However, I have a workaround you'll want to consider.
You can create a customised estimates and invoices template using a .docx file. From there, you can add a stamp or sign. Just make sure they are on neither audio, ClipArt, Word Art, animated GIFS nor embedded files cause these files may cause errors.
Once done, we can import it to QuickBooks Online so you can use it in creating transactions. We also have samples so you can start from there. For the detailed steps, you can check this article: Import custom form styles for invoices or estimates.
I also suggest visiting our Help Articles page for more tips in using QuickBooks Online.
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