Casting is to be billed out on date of casting. Many payers will pay for 29799 RT/LT or S0395 RT/LT for the casting. Then on the date of dispense the orthotic is billed out. You Mentioned L3020. I have found that many podiatrists bill this code by mistake. The most common custom functional orthotic, made in a lab off of a "patient model" is a UCB type which is L3000.
Here is how the APMA defines it: “Prescription Custom Fabricated Foot insert, each, removable. This type of device is fabricated from a three dimensional model of the patient?s own foot (e.g. cast, foam impression, or virtual true 3-D digital image). This type of orthotic is a functional device, (reducing pathological forces) which has a molded heel cup and trim lines with substantial height to provide both medial and lateral directive forces to control the hind and fore foot. It may also have intrinsic or extrinsic posts designed to control foot motion. This device is made of a sufficiently rigid material to control function and reduce pathological forces. HCPCS code L3000 includes additions such as postings, padded top covers, soft tissue supplements, balance padding and lesion or structure accommodations. Other additions may be required as well.”
Whereas the L3020 is defined as (in short) "This type of orthotic is an accommodative/functional device, which has minimal to no heel cup and is intended to control the forefoot through a Longitudinal Arch and metatarsal support."
In my experience, most, if not all custom orthotics have a heel cup. I have been to many conferences and read many articles that recommend the L3000 code rather than the L3020. Which would be billed out on the day of dispense. There are some payers, like Partnership Health Plan which is California Medicaid that wants both the casting and the orthotic billed at the time of casting/order.