To simplify I'd suggest sending your customer their record ID or Billing ID, whatever number corresponds to the actual Quick Base record #. Make this prominent in the email they get.
Then on your Decline web form ask for this ID as part of their submission.
You can then update the code on your page to use API_EditRecord action in place of the API_AddRecord and include an input field for the record ID. Defining the input as follows:
<input type="text" id="key" name="key" />
Here is a very rudimentary example where you'll just need to substitute your values:
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<form name=qdbform method=POST onsubmit='return validateForm(this)' encType='multipart/form-data' action=
https://yourcompany.quickbase.com/db/yourtableid?act=API_EditRecord&apptoken=xxxxxxxxxx>;
<input type=hidden name=fform value=1>
<div class="sub-heading">Thank you for responding to the email</div><br>
<div><textarea name=_fid_18 rows=6 placeholder="Why are you declining this bill?"></textarea></div>
<br>
<label>If you would like us to follow up, please check this box
<input type="checkbox" name=_fid_26 value="1">
</label>
<br>
<br>
<div><input type="text" rid="key" name="key" placeholder=" Billing ID" /></td></div>
<br>
<input type=hidden name=rdr value='
https://yourcompany.quickbase.com/db/yourdbid?a=dbpage&pageID=X'>;
<input class="button" type=submit value=Submit>
</div>
</form>
</body>
</html>