Galette

A quick fix would be:

diff --git a/galette/templates/default/ajouter_contribution.tpl b/galette/templates/default/ajouter_contribution.tpl
index d86b99a6..43a63228 100644
--- a/galette/templates/default/ajouter_contribution.tpl
+++ b/galette/templates/default/ajouter_contribution.tpl
@@ -160,7 +160,7 @@
     {if not $contribution->id and $pref_mail_method neq constant('Galette\Core\GaletteMail::METHOD_DISABLED')}
             <p>
                 <label for="mail_confirm">{_T string="Notify member"}</label>
-                <input type="checkbox" name="mail_confirm" id="mail_confirm" value="1" {if isset($smarty.post.mail_confirm) and $smarty.post.mail_confirm != ""}checked="checked"{/if}/>
+                <input type="checkbox" name="mail_confirm" id="mail_confirm" value="1" checked="checked"/>
                 <br/><span class="exemple">{_T string="Member will receive a notification by email, if he has an address."}</span>
             </p>
     {/if}

Note: with this fix, the box will always be checked; even if you've unchecked it when you initially submitted the form (this may happen when an error occurs saving the contribution, or if there is a contribution overflow).