Yeah, typical. We get it all the time since we made friends with a hot tub store. Here is what I do:
Flat rate for the install is $XXX.xx + $X.xx per foot to install the tub company's disconnect. (PM me for current rate). The flat price is just for a disco that can be mounted on the house and the circuit ran in the attic to a modern panel with empty space. All others, are bid accordingly.
Also, the "whip" from the disco to the tub can be connected by either us or the tub guys (and since we know them, I have no problems with them making the connection). If the tub is there when we install the circuit and disc, we make the connection. If they are going to deliver it at a later time, they can connect it, since they come out anyway and charge for a "set up" where they fill the tub and set up the chemicals.
And, we get paid from the customer, not the tub people.