List all items you wish to purchase...or
List all items that you wish to purchase.
To ensure you receive credit, please include the following...or
To ensure that you receive credit, please include the following...
I would use the second sentence in both instances because it seems to flow easier.
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It is sometimes not an easy choice, guest. If the meaning is clear without it, feel free to omit the 'that":
'List all items you wish to purchase.'-- here there is no problem of interpretation, so 'that' can be safely omitted.
'I found you have been omitting your apostrophes' -- We read and listen linearly, so the first three words, 'I found you', may temporarily mislead us; in this case, a 'that' might make the meaning immediately clearer: 'I found that you have been omitting your apostrophes.'
It is usually a matter of style, not correctness, and there will be different opinions as to the clarity in each case. As you say, it is often a sense of 'flow' and no more that will influence the choice.
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