A similar situation occurs in the world of football. There are 'season ticket holders' and fans who turn up for a match on the day.
One description of people who come in on a whim is 'passing trade'.
A measure of how many customers a shop attracts is 'foot fall'.
With mobile phones you have PAYGO -- Pay As You Go or monthly contract customers.
Similarly a large project will have a standing order with their suppliers to supply 'as and when required'.
I saw a term recently 'Zero Contract'.
Someone mentioned 'subscribers' that is the terminology of magazines and newspapers.
You could think of 'impulse buy' or impulse buyers. A similar situation arises with 'fair weather fans'.
Some goods are perishable - they have to be sold on the day, whereas others are consumer durables.
In the UK there are all sorts of sales and special offers, including the use of vouchers; they go under the name of promotions. There are also 'Trade Discounts' and special prices for multi-journey tickets.
A rather disrespectful word for describing customers is punters.