It sounds like you purchased the devices incorrectly.
If the user purchased the devices via retail sales, then your only option is to use Apple Configurator and one by one side load the devices into ABM.
If the user spoke with the Apple Business Sales teams and made the purchase through them, then Apple should be able to associate the devices to the ABM. You will need to provide Apple with your Org ID and they will provide you with your Apple customer number which you will need to add to ABM. Once these numbers are exchanges and the purchases are proven to be from the ABM legal entity, Apple should be able to backload them.
Remember, ABM is all about enforcing chain of custody. Retail purchases do not qualify for the chain as there is nothing to truly prove the purchase if for the business.
Assuming the user just walked in a bought devices, you need to guide that customer into establishing a Business Sales relationship with Apple. This can be done at the Apple Store. Ask for the Business Sales lead. Once this relationship is established, then ALL purchases should go through them. If you have another reseller or a cellular provider, then exchange your Org ID for their reseller ID to establish a link in the chain of custody.
Based on your original post, I think you have Apple Configurator in your immediate future.