Tizliz
I have always found Tesco’s receipts for online shopping confusing but yesterday they excelled themselves. I used some of my vouchers to pay - about 60% of the cost. The receipt showed every item reduced by 60%. The real confusion came on a substitution of something that was on special offer. I purchased a bottle of whisky reduced from £35 to £26, it was replaced with a bottle priced at £36 - the difference should have been £10, but it was £4 ish. However the total of my bill seemed about right. When I asked for an explanation they said sorry they had worked it out wrong and gave me 35p back.
I have no idea if the rest of my bill was right as I lost the will to live at this point.
So perhaps they would have charged you 40% of the £26, but instead charged you 40% of the £36, so then gave you back the £4, as that is the difference of the two scaled down prices, as £4 is 40% of the £10..
If that were so, then the total you got charged would be right.
Why they gave you 35p though does not fit into that. 
The online receipts go crazy in what is written, (though what is charged works out), if one has ordered more than one unit of something and they have available at least one but not all of the quantity that one has ordered and they substitute.
For example, order 10 of A and they have 7, and send them, and also send 3 of B as a substitution (as requested).
That tends to be listed as
10 A substituted with 1 B.
Then later it says under the rest of the shopping,
7 A
Sometimes with something like
Was £1.20 Now £3.60
Yes, I know, but it does! 
When the shopping arrives there are 7 A and 3 B, as there should be, it is just that the receipt is crazy.
Though the online receipts are good as they give me advance notice of any substitutions and thus makes it easier for me to make a decision whether to refuse a substitution if I want to do that.