TLDR: You do not have to attach the tax receipt to your electronic tax return, but you must be able to present it to the tax authorities if they ask for it. Without this supporting document, the tax reduction may be refused.
This is a tax reduction
For donations made to an eligible organization, the tax benefit is a tax reduction, not a deduction from income. You must therefore comply with the conditions applicable to this reduction.
The tax receipt must attest to the amount and date of the payments, as well as the identity of the beneficiary organization.
The supporting document is not attached to the electronic tax return
When you declare your donations online, you do not have to attach the tax receipts to your electronic tax return. However, you must indicate the identity of each beneficiary organization and the total amount paid to each one for the relevant year.
This absence of immediate submission does not mean that you can benefit from the reduction without supporting documentation.
You must be able to present the tax receipt
At the request of the tax authorities, you must be able to present a tax receipt that complies with the model established by the authorities. The form provided for this purpose is the receipt for donations and payments made by individuals under Articles 200 and 978 of the CGI.
Consequence in the absence of supporting documentation
If you cannot present the required supporting document, the tax reduction is refused without a prior proposed adjustment.