Every product has certain features and benefits. In helping customers come to a decision your job is to point out the appropriate benefits of the products they are considering.
A word of warning here –
Never run down one product or price to promote another. Instead point out the extra value of the alternative. Don’t say “This is better value than that because…” Your customers decide what is better value based on the benefits that matter to them.
This is the same for buying multiple copies compared to buying one.
The unit price may be considerably less for the bulk purchase but if you say “It’s a lot better value if you buy these in boxes of 20” the implication is that buying them singly is expensive. It isn’t if you don’t use many! Your role is to make sure the customer is aware of the options and point out the benefits of each. So you might say:
“If you use a lot of these you may find it more economical to purchase a box of 20. On the other hand if you don’t, they are very well priced at only $5.00 each.”