Often they have a claim form that you can use rather than submitting a receipt. You fill out the details, and then get someone at the provider to sign saying that it is accurate. As long as you have someone in administration billing you can talk to and get them to sign the claim form. Then you can avoid getting a receipt to justify the spending to dFSA custodian.
Technically, you will still need an actual receipt to satisfy the IRS if they audit you. But it's not required to file, and your supporting documentation about the details should be enough to satisfy them if you really needed to.