The purchases from the department store seem to have been your choice. Why didn't you say you didn't have the money? That's what I would have done and have done to my in-laws, especially if it's true. You buying her things is not your responsibility and it seems like you're mad over it even though it honestly is your fault, not hers or your husband's. Now, if she had forced you to purchase it or your husband pressured you to do it, that's a whole different story.
In regards to her wanting to visit her cousins: that's something you and your husband should sit down and figure out the budget for what you can and can't afford. Perhaps he's upset that you immediately seemed hesitant over it instead of waiting to check if it's even possible. You say you're on a tight budget, which makes me believe you have a spreadsheet laid out showing what's coming in and going out, well... pull it out and talk with your husband about the numbers. Numbers can't lie and if it's something you can't afford, but he still insists on then you know he's not being reasonable and is putting your personal finances in trouble. Personally, if my husband continues to insist in something we can't afford this would be grounds for a serious conversation about communication and respect. Parents are important, but their whims shouldn't be more important that your marriage's finances. Good luck!