It is very risky business to offer these services.
Customizations are generally done under the condition that the wig cannot be returned after being altered. Clients are often eager to take advantage of these services and dismiss the risk that they may not like the outcome...until they realize they cannot return the wig! So, if they receive the wig and it doesn't work, there is generally a very high likelihood that the dissatisfied client will file a chargeback to get out of paying for the services. Chargebacks are very damaging for small businesses, so most of us try to avoid them at all costs.
Most customizations require not only a competent hair stylist who knows what they’re doing with all types of hair (which is extremely rare, since they don’t cover how to cut and handle synthetic or heat-friendly synthetic hair in most beauty schools), but the person altering your wig must also be an expert seamstress. Even slight cap alterations require expertise with a sewing machine, needle, thread, what-have-you. This becomes even more complicated when a specific fit requirement is the impetus for the customizations because the risk of mistakes in the customization process can increase exponentially if the person making the alterations never gets to see how the wig actually fits on your head prior to cutting or sewing. All kinds of things can go awry.
Since we don't like chargebacks and y'all don't risk, we don't offer these services.