On the Snap account summary page there are two options for buying extra data if you're likely going to go over your limit - 'purchase additional data' and 'additional data packs'.
'Purchase additional data':
And 'additional data packs':
The second one is obviously far better value than the first, both can be added at any time, and the second one can be set to be removed at the end of the month. So with the possible exception of the $2, $5 and $10 ones, why would you ever pick the first option? Genuinely struggling to understand the reason for the two very different offerings.